I have not played the Bernolin resin recorder for any extended lengths of time, so I cannot answer you on that. But I can tell you several things.
First, they clog about as readily as regular plastic recorders, so they benefit greatly from the use of his fluid. As far as his fluid is concerned, I think it is VERY effective, not just in terms of preventing condensation clogging, but also longevity-wise. One treatment lasts me about 3 or 4 weeks. I suppose it depends how much you play, but it is MUCH more effective and longer lasting than stadard treatments I have tried in the past.
On the fluid page of his website, he say, “Can all recorders enjoy LM77? Yes. Whether plastic, wood or resin, or standard LM56, all instruments can be treated with this product. However, it should be noted that the alcohol contained in LM77 can discolor some woods like rosewood, blackwood, or tinted recorders. It is therefore imperative to avoid drips on the outside of the instrument during the application.”
I have not tried it on anything other than the resin recorder, but I don’t see any reason why it wouldn’t work well on other types.
I think when he says you can play it for extended periods, he may be talking moire about what is SAFE for the recorder itself, not necessarily how well it will play. I mean, a wooden recorder has wood throughout and gets saturated with moisture very quickly when you play, throughout the entire instrument. This will not adversely affect the resin recorder at all, whereas if you play for extended periods, a regular wooden recorder can crack, warp, split, etc, etc. For example, the barrel of a recorder can easily warp into a slight curve.
Also, the cedar block in the resin recorder is surround by resin holding it in place. Yes, it swells and changes over time. For example, on mine it keeps moving outward, so occasionally I have to push it back in, or if it needs more of a jolt, I remove it and put it back in to get it properly positioned. But the rest of the windway doesn’t change, of course, so I think it helps keep everything in order, square, and fitting OK. I have never had a block be hard to remove or put back (after letting it dry, of course).
As far as playability in an extended session, I did hear one person complain that after a few hours it didn’t play too well, but that is just an isolated case. I can imagine that what you said is correct – after awhile, even it will be affected by the block swelling in that one session. But at least you won’t have to worry about damaging the recorder.