Sorry, I haven't used the "JDEGTAddMiscBlob", so I have no direct answer to this question.
Yet, OAM does offer a solution and as you already have it, you can just plug it in and use it right away - I assure you that this would be less complex that trying to figure out some undocumented API's (which may also not work at all - there are quite a few dysfunctional "stubs for the future" in there).
I understand your reaction, but OAM _is_ a possible solution, even if it's not a _direct_ answer to your question. And if I never pointed this out, you probably wouldn't have it now, meaning that you would likely have no solution, for whatever original issues you are using it for, at all. So generally, I think it's a good thing that I occasionally do.
Under XE, the existing API's are few and far apart, so I was using lower-level API's, like JDB_xxxxxx table access API's, and did all additional processing outside of JDE, so there's no simple answer...
The official answer for XE/ERP8 users from JDE has always been "mal fortuna", although they did introduce a number of new API's in the last few Unicode versions of JDE (albeit still undocumented and consequently barely useful)...