Thanks guys, IIc is back in 'no solution' land
The Apple IIc reference manual seems to suggest you can boot a ProDOS disk from external:
"The PRODOS operating system (but NOT the DOS or PASCAL operating
systems) supports startup using the external disk drive. This
PRODOS feature makes it possible to start the Apple IIc with a
diagnostic program in the event that the built in drive does
"To restart using the external drive, insert a PRODOS disk in
the external drive, then invoke the Monitor (CALL-151) and
issue 7, CONTROL-P."
I imagine this would be the same with the floppy emu and a mounted 5.25 prodos disk.
EDIT: Nevermind. Seems the only ROM version that allowed this was the 255 (first?) revision, and even then most disks would try to later seek the internal drive. This taken from BMOW:
IIc ROM Versions
For the curious, you can type PRINT PEEK (64447) at the BASIC prompt to find what ROM version is present in an Apple IIc. There are four versions:
255 The original Apple IIc ROM, size 16K. No support for Smartport disks. However, it has a nifty PR#7″ feature that allows for booting from an external floppy drive configured as drive #2, something thats not normally possible. This was the ROM installed in my IIc. In my tests, PR#7 worked to begin booting from the external drive, but most software seemed to be hard-coded to expect to boot from drive #1. The disks would start the boot process, but then the empty drive #1 would begin seeking in vain. The only disks I found that booted successfully from drive #2 were ProDOS and the game Moon Patrol.
0 With version 0, the ROM doubled in size to 32K. This ROM version eliminated the option for booting from drive #2, but added support for Smartport disks. The most common of these was the Unidisk 3.5, an intelligent external 800K floppy drive. But the Smartport protocol could also be used to communicate with other types of external disks, including hard disks up to 32MB in size.
3 Version 3 accompanied a logic board change, making it possible to expand the IIcs memory.
4 Version 4 corrected some bugs present in version 3.