nightstalkerpoet said:

Create the avchd/bluray with tsmuxer, then load the folder into multiavchd.

The other option could be bdtoavchd, bdremux mode

Thanks for that nightstalkerpoet, very helpful indeed!

I can't get a handle on multiavchd, it seems to require reencoding whatever I do but I have managed to get bdtoavchd to work in bdremux mode like you said, leaving both video and audio untouched, and it allows the forcing of the first subtitle without burning it in.

Only one problem though, it just supports one subtitle. I do believe I have got round that though.

If you take the avchd created by bdtoavchd and open it in tsmuxer and leave the first subtitle as it is, it seems you can add more subtitles and the first stays forced when you create the avchd again in tsmuxer, so you get the first subtitle forced and also a choice of multiple subtitles. I just need to find a dvd9 to test it out with now, to see whether it all works as it should.