We use a dedicated printer in SAP for these slips, rather than front end printing, so I haven't hit this problem. Could you do the same thing? The config gets a bit more complicated if you want to print to different printers from different locations, but you already know where to do that - you just need multiple SAP printers, and use different devices from different storage locations. As far as I know, output determination is the only place you need to change. But I'm not an MM expert - I just play one on SCN 
I can't immediately think why what you've done wouldn't work, but maybe this is a workable alternative?
Steve.