My question would be, how to-the-letter-of-policy are they in terms of even sending a warning (outside of obviously egregious overages)? And does someone in the process look at exactly where the roaming occurred?
The idealist in me would like to think that if you say doubled the supposed limit for example (which would be 600MB on the old plans such as I'm still on) because you were traveling in an area in which there was simply no other choice but roaming, they'd let that slide and wouldn't even send a warning. Or even if it were more than that, to weigh your native on-network usage versus what you've done vs. roaming. Obviously if someone is consistently in roaming coverage comparatively, it would be hard to justify keeping them on, but it would be nice if there were some sort of logical analysis in the mix before someone was just arbitrarily warned much less dumped.
Back to the OP, maybe they could institute some sort of dual-cap hybrid scenario that incorporates the OP's suggestion, i.e. roaming is free up to 250MB per month. Past that, its X amount per megabyte which is very reasonable/competitive and not hurt those that may travel to areas with only roaming as an option much, but those rates only apply up to 1GB of usage, at which point the per-megabyte charges jump to hefty/significant per-MB charges past that.