I don’t understand what you’re doing here. The BIP you link seems to only talk about plain OP_TRUE, right? So the malleable version? What part of the BIP141 are you using to do this?
I don’t understand “otherwise undefined witness program”. You mean v1 programs that are not 32 bytes and redefine those? But then why would you need OP_TRUE? Can’t you simply standardize a v1 program that is 1-byte in size and make it standard if thar 1 byte has a fixed value? Which is anyone can spend anyway, so it’s the same as OP_TRUE?