Replying to my own questions now that I have more data: yes, the DOT's interpretation of rule 250 is that you get compensation for the remaining flights coupons (=from the point of interruption).

I still don't think it's particularly fair -- a four hour delay is a four hour delay, no matter where it starts. And given how that can leave some people getting tiny bits of compensation (e.g. having flown transcon, only to get bumped before a final short-hop connection), I'm going to try to push this a little harder. Anyone interested, feel free to PM.