You could try USER0. The problem here is, that I have to register the extcap interface with the other interfaces at some point, and usually this happens way before the capture starts (at which point an extcap utility might know which DLT it could produce). Therefore I first ask the utility for all interfaces and then for each DLT per interface.
What you could do is provide 5 interfaces, one for each individual DLT. This clobbers up the interface list a little bit, but at least you would be able to use the correct DLTs for the interfaces. Combine this with a preference which interfaces should be supported, and you could reduce the clobber a little bit.
We actually provide a plug-in with our extcap utility, which allows the user to set more generic options via a gui hook-up (using the plugin_if functionality). But you could also use a more generic preference in Wireshark itself and read from there.
regards,
Roland