Hi Marc,
If there is only one pool member not marked down in a pool, I don't think the slow ramp option would do anything. I believe this option dynamically lowers the priority of the newly added pool member. If there is only one pool member available there wouldn't be other members to use more heavily.
You could potentially handle this scenario in an iRule using HTTP request limiting if [active members [LB::server pool]] == 1:
There are quite a few different HTTP limiting rules in the Codeshare. Try searching for limit on the page:
http://devcentral.f5.com/Wiki/default.aspx/iRules.CodeShare
Aaron