Probably not. MILCON projects are specifically authorized. There are no "go build good things" authorizations. Let's say some Army post needs a new Divisional HQ. There would be a specific authorization for the design and construction of the facility.
I've engaged in some scenario driven thought processes; really gaming it out formally. If the POTUS deploys Reserve Forces to actively protect the border, then Operations and Maintenance funds could be used to erect structures to support that.
The problem with O&M funds is that expire after one year and Congress could put limitations on funds the following fiscal year. That would devolve into shit fight, with those in favor of border defense say Congress is leaving our troops and population defenseless from foreign marauders and the like.