I think I just figured out how to deniably proxy-war.
The Romulans need cargo ships. The Yrillians have a very large number of them, run by independent crews looking for profit and adventure. We broker some kind of deal, and word it so that the goal of helping the war effort is clear.
Once they are in Romulan space, if some of them get a bit too enthusiastic (as Yrillian work gangs inclined towards adventure in space often do), it's no one's fault in particular, and almost to be expected if the Romulans start offering letters of marque, given the relevant cultural factors.
We could even see what we can do to increase survivability for these brave spacers risking a war zone to keep the Romulan people fed. They certainly shouldn't be showing up in ex-Starfleet hulls, but there may be some obsolete or even impounded foreign hulls we can auction off that are a bit tougher than what they might otherwise have access to, and we could revisit those self-contained weapon pods for freighters that we were working on during the biophage war. I'm pretty sure there's even a couple of mark 1 Klingon BoPs floating around taken from renegades and pirates over the years...
All that aside, I rather like the idea of using a Dreamer task force to protect them from reprisals, keep an eye on things out that way and force the Breen to keep forces back from the front. It's a really nice N-birds with one stone solution, and justifies a task force most of us want but haven't been able to make fit with everything else we need to be doing.