Is it possible to have a listing on GLBSE that can be repurchased by the issuer on a ongoing basis instead of on a particular date?
The asset still exists after a buyback if that's what you're asking. Something like how PPT.A-E operated? They would each sell a number of shares and then one month later buy all the shares back, then the asset would be recycled and used the same way next month (I think they actually reused them the next day in most cases).
If you're asking if you can buyback a portion of the outstanding shares: I'm not sure, but I believe it's possible. Hopefully someone who knows can clarify if this is what you're wondering.