@thomas
Part of your code worries me:

		dl = LockDosList (LDF_ASSIGNS|LDF_READ);
		if (dl = FindDosEntry (dl,"C",LDF_ASSIGNS)) /* because C: may be a multi-assign we cannot just Lock("C:") */
			{
			cmdlock = check_dir (dl->dol_Lock,command);
			for (al = dl->dol_misc.dol_assign.dol_List; !cmdlock && al; al = al->al_Next)
				cmdlock = check_dir (al->al_Lock,command);
			}
		UnLockDosList (LDF_ASSIGNS|LDF_READ);

Specifically I am worried about the check_dir() calls, which uses CurrentDir(), Lock() & Examine(). Unfortunately AmigaOS4's autodocs say the following about using what is returned by FindDosEntry():

/*
* Add this entry to your private list of volumes to process later
* or break out of the while loop.
*
* For simple processing, you could also use it inside here,
* but for anything else I strongly advise against it unless you
* understand the semantics about avoiding semaphore deadlocks.
* It is never acceptable to call functions that involve sending
* dospackets to any handler currently in the doslist while the
* doslist is locked.
*/

This is more stringent than the AmigaOS3 autodocs, which advised against it "for compatibility reasons", but then go on to suggest trying it anyway to see if it works(!).

However, I'm not sure how to follow AmigaOS4's advice, because it's not clear that I can keep a copy of dol_Lock or al_Lock & expect it to work after unlocking the list...

@gazelle
I've attempted to use SearchCmdPathList() to implement my own version of "Which", but it crashes. EDIT: I suck. Forgot to convert BCPL address to proper address.

SetFileSysTask: I don't see a reason for any application whatsoever to call this function. Which reason do you see?

Other code I've seen using GetDeviceProc() also uses SetFileSysTask(). Example code I've recently seen from the main developer of AmigaOS4's AmigaDOS also used SetFileSysTask(). And when I previously wrote code without SetFileSysTask() it didn't always work as expect (I don't recall details).

My vague understanding is that a lock of 0 should NOT point to SYS: when it is in the context of a device handler. Don't ask me why! My own source code comments don't say where/who I got this info from.

I would also point to the AmigaOS wiki, which currently seems down so here's Google's cache of it:
http://webcache.googleusercontent.com/search?q=cache:http://wiki.amigaos.net/wiki/Multiple_Assigns&strip=1
It contains the following code fragment:

         old_fsport = SetFileSysTask(dp->dvp_Port); /* in case dp->dvp_Lock is NULL. */
         old_curdir = CurrentDir(dp->dvp_Lock);     /* Lock() locks relative to the  */
                                 /* current directory and falls back to the root of  */
                                 /* the current file system if dp->dvp_Lock is NULL. */
 
         lock = Lock(rest_of_path,SHARED_LOCK);

This may provide a partial explanation.

Anyhow, how shall a 0 lock appear in a C: multi assign?