I've used a workgroup name with hyphens (-) for over 10 years. It's legal according to Microsoft (http://www.microsoft.com/technet/prodtechnol/windows2000serv/reskit/w2000Msgs/4225.mspx?mfr=true), but firmware 02.0.15 and 02.0.18 won't allow entering it from the web interface. Apparently an input validation routine was changed, because I was able to successfully configure my 01.01.18 MBWE with no problem.
No fears, however, in that you can vi /var/oxsemi/network-settings, put a valid workgroup name in and reboot. Samba can handle it just fine.