Crystal tickets can be added to your BAEC account in the normal way, same as any OTA, add a booking in MMB and from there add your BAEC number. You can do seat select, you get your airport benefits. You won't get POUG offers, you can't upgrade with Avios, you may find it difficult to get a refund (no experience with Crystal who may well be fine, this is general OTA experience). You will get Avios appropriate for your status level and TPs. Crystal do incidentally offer discounted J tickets too.
One thing worth remembering is that these are cheap because the underlying BA ticket is cheap. For an extra tenner or so you can book direct. It's just a lot of fun having a really cheap flight, it sets off the silly grin reflex.