I think that the 82901m is just an hpib device; that should mean you can connect it to the HP87 HPIB interface via an hp 10833-type cable (10833a, 10833b, 10833c... different lengths, the "a" being the shortest).
The CP/M subsystem, I think, is MORE than a ROM-- it is a module that LOOKS externally like a packaged ROM module, but it is really a slave processor (a z80), so that stuff written for CP/M will have a native-mode cpu to run on. That is very cool-- opens up a new door of software for you to explore-- but in order to use this module, I do believe you are supposed to use a supplied BOOT diskette that contains CP/M 2.2 by Digital Research Corp.
Somebody probably has a copy of this boot diskette somewhere or would be able to point you to the right place to find it.
CP/M on a HP87 sounds like it would be fun. The '87 would make for a great terminal. Since DR bit the dust, Caldera (I think) retained the software rights, then put it in to public domain a few years ago, including even some of the source code. I remember a few CP/M related mirrors having it around. Crawl the web and see what you can find. The tricky bit might be finding a way to get it onto the 82901m format, with the HP module calls, so you can boot it up. But, I suspect, if you put your need out, someone can help with a copy of theirs to boot up with.
Also, somebody out there may have docs for your CP/M module. Docs are always useful w/ hp stuff.
Best luck.