Veit Kannegieser's RXUNLOCK library has the RxBldLevel() function.
Interestingly, it occasionally finds BLDLEVEL signatures in executables where IBM's BLDLEVEL doesn't. I guess it uses a more tolerant search mechanism (TTBOMK it basically does a brute force search for the BLDLEVEL header through the whole file).
In theory, I think the BLDLEVEL signature can be anywhere in the file. In some cases when using REXX2EXE, I've found simply doing a binary append of a prebuilt BLDLEVEL signature to the EXE after the fact has worked. e.g.
@rexx2exe blah.cmd blah_.exe /2 /k
@copy /b blah_.exe+blah.blv blah.exe
(where blah.blv is a text file containing the pre-formatted BLDLEVEL signature).
OTOH, this technique does not work for some other executable types, like VX-REXX apps. It probably has something to do with how the executable wrapper works. (I've never tried this trick with an already-linked object code a.k.a. 'native' EXE.)