So that’s a no then 🙂 Thanks for the advice and comments. I can see why you might not want to add layback if the post wasn’t designed for it, although the fact that the i-beam versions of the GD do allow it would suggest that you’d be unlikely to kill the post if you did.
To be honest I’m not actually sure that the bike is too small. As well as the (very old) 18″ Five I have a 20″ Trance. One of them will go once I can work out which I prefer, but I can’t even decide which size I prefer 😳 For riding along a road, fire road or smooth trail the larger Trance feels better, but once things get a bit more technical the relative ease with which the smaller Five can be chucked around is a big advantage. Given that these bikes are purely for fun I’m not sure that I would want to give up that manoeuvrability just to get a bit more long distance comfort. Would be nice if I could move the seat back a bit though.