On the Tin and copper (and all members, really), it seems like you can get a 25%-400% (depending on what you’re doing) effective belt length increase if you can get the specializer to add something like conform ops every couple of instructions to clear dead values or move useful ones to the front. If 80% of values are only used once on average, it makes sense to get rid of them asap, leaving more space for useful values. The specializer should already know live/dead info, since it has to figure out whether to spill things. The question is whether you have enough holes in the schedule to take advantage. It might be worth adding conform to other slots or adding a few bits to the encoding specifically for this purpose. It’s extra hardware, but maybe less than a longer belt?
You could also do something goofy like automatically killing belt values that get spilled or written to memory. You would have to add additional ops for cases where you don’t want that behavior. I have no idea whether this would be worthwhile, though.
Of course, this assumes you usually have at least a few dead values on the belt.