*CIMPLODE
... generates an error on the BBC Master series.
*CEXPLODE
...generates an error on the BBC Master series.

58 ("&3A")
Read a byte from memory (intended as an alternative to "OSBYTE &05").
59 ("&3B")
Write a byte to memory (intended as an alternative to "OSBYTE &06").
(NOTE: This uses the "OSBYTE &3A" code for reading memory - see the
details above for more information.)
The "32-bit logical address" system used by "OSBYTE &3A", "OSBYTE &3B" and
"OSBYTE &3F"

&FFEn8000 - &FFEnCFFF Sideways ROM/RAM ("n" = Sideways ROM/RAM #)

As I've run into difficulty calling "OSBYTE" from within these routines ...it looks more like the MOS "OSBYTE" and "OSWORD" handlers aren't re-entrant...

I'll look to do this coding for a future release

Some notes:
No, should just return silently.

You mean "OSWORD &05", "OSWORD &06", "OSWORD &3A", "OSWORD &3B", "OSWORD &3F". Also, check you're not clashing with other OSBYTE/OSWORDs, check the lists at http://beebwiki.jonripley.com/OSWORDs and http://beebwiki.jonripley.com/OSBYTEs and propose any new allocations on the Discussion pages and the BBC-Micro mailing list.

It would be preferable if you used &FFFRxxxx for sideways ROM addresses to be consistant with everybody else: http://mdfs.net/Docs/Comp/BBC/MemAddrs

Correct, they aren't, you have to save and restore the parameter data at &EF,&F0,&F1 around calling OSBYTE/OSWORD from within an OSBYTE/OSWORD routine.

I'll have a quick look at the code, see if any other gotchas jump out and give you any pointers.

Also, on the subject of assembling large amounts of code (eg sideways ROM images), I wrote *Assem years ago to do just that, it assembles from files on disk using the BASIC assembler. It needs documenting a bit better, but it lets me write multiple-file BASIC source like this.