I must be missing something - the standard NUnit GUI runner allows you to load multiple assemblies and run them as a batch.
Update 16 July
Apologies, I'm guilty of not reading the question properly. Still, I hope the following will be helpful.
I'm assuming that you have a (possibly large) collection of test assemblies, and want to avoid having to load them all manually each time you run tests. NUnit "project files" can help.
Within the NUnit GUI runner, select File|New Project
and create a new .nunit
file at the top of your projects directory structure. Add each of your test assemblies to this project (Unfortunately, this seems to be a one-by-one task, as there's no search nor even multiselect). Once you've added all assemblies, save the project file.
Next time you want to run your tests, you can just load your .nunit
file and all of your assemblies will be picked up for testing.
This isn't as convenient as a wildcard, and it does require a little maintenance, but does avoid having to manually select a whole bunch of assemblies every time.