SOAP is quite specific and critical to data format and structure.
You might extend it, but then Edge Server is the active part and you depend on its regular "broadcast".
If Doc Server should be the active partner I'd rather suggest to use JDBC in Linux.
Your actual $get might be embedded as a Method projected as SQL Procedure that you call.
There exist examples to even execute COS commands by that approach:
see: ObjectScript over ODBC

if it is a straight 8bit DB  you have the challenge to find out
how characters beyond the primitive 7bit ASCII are stored  e.g. ÀöÜß € ¡¿ è & é ç
is it ISO Latin 1 , 2 ,3 4, 5 ... or any of the horrible Win encondings

2 mistakes: 
#1^$get(node) delivers the content of variable node
but you would expect the result of $get(@node)  using node as global reference

#2) the better way is to use the implicit GET
Set node = $Query(^FromExtraMed(""),1,value)
write value