While embedding root-filesystems to create a live-USB using paqfs I found some interesting behavior :

Starting your user file server with

*argp++ = "userfsrv";
*argp++ = "-S" ;
*argp++ = "ufsrv" ;
*argp++ = "-f" ;
*argp++ = partition ;
for(i=1; i<bargc; i++)
*argp++ = bargv[i];
*argp = 0;

exec("/boot/userfsrv", arg);
fatal("can't exec userfsrv");

and using :

int nfd=open("#s/ufsrv", ORDWR) ;
return nfd ;

works, while using -i to connect over stdio (-pipes) does lead to a version error in devmnt

Using this method its quite simple to create a bootable USB (install-) live Stick from within 9legacy or 9vx.

I didn't have the time to search for the reason behind this changed behaviour in devmnt.

With this workaround I created a bootable install CD from the 9legacy ISO for devices which don't have CDROM devices.