With my switch, I could configure trunk port the way you've mentioned in the first post:
interface 0/50
vlan participation include 10,11
vlan tagging 10,11
I could issue a "switchport mode" but couldn't go further. Looks like a bug.
For my switch version ( LB4M 48x1G 2x10G, 1.1.1.8, VxWorks...