> I'm looking for pointers/hints/documents/samplecode/anything about how to 
> create a combine message for a devctrl call.

Perhaps you could you clarify what you are wanting to combine?

Combine a devctl with an initial open to get a more efficient pathname-targetted call?  Look at lib/c/1/stat.c (and 
replace the IO_STAT msg hdr with a IO_DEVCTL one).

Combine devctl with other messages?  Combine multiple devctls?  Somethign else ... ?

How to handle it in the client?  You must hand-craft the message.

How to handle it in the resmgr?  Generally you do nothing special, the resmgr layer will make your attached callout for 
every combined message structure it finds, and usually you don't care either way (unless avoiding reply data, eg see lib
/c/iofunc/iofunc_lseek.c).