I'll answer my own question.

It is true; the last arena takes the remaining space. This is not good if
the intention is to dump to fixed size removable media, which is the
whole reason for the arenas in the first place. I know that if the last
arena is filling, then it's time to get a new disk drive, but it would be
nice to be able to dump the last arena before getting a new disk.

This is the code in fmtarenas..

		if(limit >= ap->size || ap->size - limit < MinArenaSize){
			limit = ap->size;
			if(limit - addr < MinArenaSize)
				fatal("bad arena set math: runt arena at %lld,%lld %lld\n", addr, limit, ap->size);
		}

Now I think that should be

		if(limit > ap->size || ap->size - limit < MinArenaSize){
			limit = ap->size;
			if(limit - addr < MinArenaSize)
				fatal("bad arena set math: runt arena at %lld,%lld %lld\n", addr, limit, ap->size);
		}

anyway.

I'd suggest replacing this with

		if(limit > ap->size)
			break;