As a first step, I figured out how to run mayfly under jruby. Here's what I did.
First, I installed jruby JRuby 1.1 RC 2as directed. Then, I put the mayfly 0.3 jars in $JRUBY_HOME/lib (there are other ways to tell jruby where to look for them, but this seemed like the simplest).
Then, I put the following in hellodb.rb:
d = Database.new()
d.execute("create table foo(x integer)")
d.execute("insert into foo(x) values(5)")
d.execute("insert into foo(x) values(6)")
Then running jruby will invoke mayfly:
$ jruby hellodb.rb
The next step would be to figure out how to get Active Record talking to Mayfly.
Haven't tried that yet.