I dunno if this'll work for your purposes, but what
I'd do is create the
proper tempo map in ardour, create a beat in hydrogen of the right tempo
for the section, export it as a .wav and import it into ardour - that's how
i do all my hydrogen stuff with ardour
but this way you can't simply change things as you like in realtime, i don't
like this. sync out/in has to work so that both programs can be used in
realtime rather making wave files each time which is ok if you are doing your
final mixdown.
How are you going to anyway? Hydrogen kind of wigs out when you start
altering the tempo during play. Doing it in ardour would probably be
too slow; then again it might actually work if you did it far enough
ahead...I don't know. Usually though you have some idea what the
tempo is before you record. When would you use 'realtime' tempo
changing and how would it work?