This can be accomplished, though it would require multiple specific events. Here's my take on it:
First the obvious one: Have early 18th Century Russia invest heavily in North American expasion, moving the Alaskan border progressively south until it hits California. Perhaps some change in imperial policy, coupled with some very good harvest years, a steady economy, and a desire to prove itself a even greater Empire by carving out a piece of the New World. With a steady and growing russian presence in OTL Alaska, BC, Washington and Oregon, we now have a major barrier to British and American expansion in the northwest. I imagine that if Russia really put it's mind to it, the prospect of settling in fertile Oregon would be very attractive for your siberian Boyar.
Now, a VERY different Mexican-American war. The United States carves out the Mexican gulf for itself, moving it's border waaay south, while Mexico would keep it's western territories(including California, Arizona, New Mexico, Utah, etc.). The U.S. then supports a Independent Yucatan Peninsula Republic as a nice Caribbean "partner". Yucatan could expand northwards beyond it's Peninsula to border the U.S. at the Gulf.
So now you have Russian Northwest / British Canada with no Pacific access / United States minus the west coast, but with a huge chunk of eastern Mexico, almost surrounding the Gulf / Mexico, oing all the way from California in the North to it's present southern border, but without it's eastern regions and Yucatan/ Yucatan Peninsula.
There. Not Unthinkable. The US southern stretch will look ugly in a map, but I can imagine worse.