In case anyone is interested, I've attached my port of MIKBUG REV. 9 to the 6800 Apple-1.
This version is adapted to use the RAM-based IRQ vectors in my variant of a1mon68, but it should work with unmodified a1mon68 as well, as long as IRQs (e.g. SWI) are not used.
Using the `L` command it's possible to load software from S-Records like the TSC 4K Micro BASIC Plus 4K variant found here (manual), which seems to be set up to use the MIKBUG entry points for I/O, though I'm not sure it's designed for this version so I don't know how well it will work without patching (I've been able to load and start it at entry point $0100, type in and run a simple Hello World program, but that's about it).
It's also possible to load the version of the co-res assembler found here (reference 1, reference 2).
This actually works (tested under an emulator); I have been able to use it to assemble code successfully.
If the system has RAM beyond $7FFF then a patch is needed to adjust the address at locations $53,$54 to avoid symbol table overflow error (221). Also, for Apple-1 the $7F (DEL) at $10F9 should be replaced with $0D (CR). The following patch (to be loaded after the co-res S-record) does both of these things, and also sets the PC to the warm-start entry point so that the `G` command will avoid initialization that undoes these changes.
The procedure for loading this from MIKBUG is:
Here is the source for the preceding co-res patch (which can be assembled with the co-res assembler itself). I had to recreate it, as I had lost the original file.