Need to decompress string compressed with zlib. Can I manually add the "customized wrapper" intersystems requires for $System.Util.Decompress()

If I understand correctly you need to deflate and I suppose you want to add the custom wrapper to use $System.Util.Decompress(), if so...you can't. I mean you can't use $System.Util.Decompress() BUT, you can indeed deflate using I/O as in the example I posted in that post/thread.

Here is a sample method to deflate without using file I/O and using XDEV instead.

ClassMethod UnDeflate(Compressed As %String, Output UnDeflated As %String) As %Status
{
    Set sc=$$$OK
    Try {
        ; open XDEV device
        Set dev="|XDEV|"_+$JOB
        Open dev:($ZF(-6,$$$XSLTLibrary,12):/HOSTNAME="XSLT") Use dev
        ; write compressed string
        Write Compressed
        ; flush buffers
        Write *-3
        ; change device mode to deflate and rewind
        Use dev:(:/COMPRESS="DEFLATE":/POSITION=0)
        ; read uncompressed content
        Read UnDeflated
    } Catch CatchError {
        #dim CatchError as %Exception.SystemException
        Set sc=CatchError.AsStatus()
    }
    Close dev
    Quit sc
}

I really wish that InterSystems implement the various compress/deflate/gzip functionality available in device I/O as utility methods. Without silly wrappers! 😉

Another wish, please InterSystems document XDEV device I/O.