The simplest way is to create a classmethod, which returns the desired name:

Class DC.Evgenys.Data Extends %Persistent
/// which: 0=Fullname, 1=Schemaname, 2=Tablename
ClassMethod TableName(which = 0) [ CodeMode = objectgenerator ]
    set sch=%compiledclass.SqlSchemaName, tab=%compiledclass.SqlTableName
    do %code.WriteLine($c(9)_"quit $p("""_sch_"."_tab_","_sch_","_tab_""","","",which+1)")
    quit $$$OK

So you can do something like this:

for i=0:1:2 write ##class(DC.Evgenys.Data).TableName(i),!

A possible work-around could be the class below. In short, you work with your json property as intended, merely before saving the object, you save the json-property into a stream and after opening an instance, you restore the json-property from the the stream - that's all. The drawback, no SQL over the json property...

Class DC.Dyn Extends %Persistent
Property json As %DynamicObject [ Transient ];
Property jstr As %GlobalCharacterStream [ Internal, Private ];

ClassMethod MyTest(kill = 0)
   if kill do ..%KillExtent(1,1)

   set obj=..%New()
   set obj.json.short="A short test text"
   set obj.json.maxstr=$tr($j("",$$$MaxStringLength)," ","X")
   do obj.json.%Set("hugedata",,"stream")

   write "Status : ",obj.%Save(),!
   set id=obj.%Id()
   write "ID : ",id,!
   kill (id)

   set obj=..%OpenId(id)
   write "short : ",obj.json.short,!
   write "maxstr : ",$e(obj.json.maxstr,1,20),"... Size: ",$length(obj.json.maxstr),!
   set stream=obj.json.%Get("hugedata",,"stream")
   write "hugedata: ",stream.Read(20),"... Size: ",stream.Size,!

ClassMethod stream(obj)
   set stream=##class(%Stream.TmpCharacter).%New()
   do stream.Write(obj.json.short)
   do stream.Write(obj.json.maxstr)
   do stream.Write(obj.json.maxstr)
   quit stream

Method %OnOpen() As %Status [ Private, ServerOnly = 1 ]
   if ..jstr {
      do ..jstr.Rewind()
      set ..json=##class(%DynamicAbstractObject).%FromJSON(..jstr)
   Quit $$$OK

Method %OnAddToSaveSet(depth As %Integer = 3, insert As %Integer = 0, callcount As %Integer = 0) As %Status [ Private, ServerOnly = 1 ]
   do ..jstr.Clear(), ..json.%ToJSON(..jstr)
   Quit $$$OK

Some testing...

IDEV:USER>d ##class(DC.Dyn).MyTest(1)
Status  : 1
ID      : 1
short   : A short test text
maxstr  : XXXXXXXXXXXXXXXXXXXX... Size: 3641144
hugedata: A short test textXXX... Size: 7282305

If your code uses obj.%Reload() then %OnReload() and %OnOpen() should contain the same code.

Your solution is just perfect. And fast.

But yes, you can avoid string manipulations... This one, for example, uses math only, merely it's neither short nor looks elegant:

 set dt=$h write $zd(dt,8)*100+($p(dt,",",2)\3600)*100+($p(dt,",",2)#3600\60)*100+($p(dt,",",2)#60)

but gives the same result as your short and nice solution...

 set dt=$h write $zd(dt,8)*100+($p(dt,",",2)\3600)*100+($p(dt,",",2)#3600\60)*100+($p(dt,",",2)#60),!,$tr($zdt(dt,8)," :")

On the other hand, you can install new brakes on your car, as suggested by others... ;-))

Just compare those codes with yours:

set h=$h, t=$zh for i=1:1:1E6 { set x=$tr($system.SQL.TOCHAR($h,"YYYY^MM^DD^HH24^MI^SS"),"^") } write $zh-t,!

set h=$h, t=$zh for i=1:1:1E6 { set x=$tr($zdt(h,8)," :") } write $zh-t,!

The choice is yours...

Just to put things in right perspektive, those "one letter commands" and "a lot of them in the same line" were neither tempting nor addictive, they were simply necessity!

At the time of the birth of MUMPS (the core of Cache/IRIS/etc.), more then 50 years ago in the second half of 1960es,  memory (which was a real core memory at the time) was rare and expensive and was measured in units of kilobytes! Just to contrast, today's server have the same amount of RAM, but in gigabytes, that's a factor of one million!

As a consequence of memory shortage and because MUMPS of that time was interpreted (i.e. you loaded the sourcecode into memory), one had to utilize each and every possibility to save memory. One of those possibilities were the ability of the language to short each command to one letter and to put as many commads as possible into one line (thereby saving line-ending bytes).

The tools (to save memory) of that (ancient) time were argumentless IFs and ELSES, short (variable-, global- and routine) names, commands with postcondition and sophisticated programming.

Last but not least, if one aims to "modernize" thos old applications, should be keept in mind, especially, if one is not so familiar with the old fashioned style and methods, there will be many unexpected pitfalls.

Sample1: on old printers, the line with "Total..." will be printed "bold-alike"

 write "last item",?15,$j($fn(val,",",2),10),!
 write ?15,"----------",! do  do  do
 . write $c(13),"Total",?15,$j($fn(sum,",",2),10)
 write !!!,"Due date for payment ....",!

Sample2: converting from:

 ; normal flow
 . ; nested
 . ; commands
 ; normal flow


 ; normal flow
 if 1 {
   ; nestd
   ; commands
 ; normal flow

will be in most cases OK, except, if the nested part uses the current value of $STACK:
this is now one less then in case of argumentless DO!