Keuchel has an option left. With 5+ years service time, we'd need his consent to use it, but it should be pretty straightforward to get him a couple minor league starts if/when he signs. The Braves optioned Ervin Santana with his consent to begin the 2014 season, to make 1 start at AAA, after he signed late (March 12, about the same point as Lance Lynn in 2018)
(As an aside, we could have done the same with Lynn, as he has 2 option years left -- although I feel like his late start was perhaps more of a convenient excuse than the primary driver of his 2018 issues.)
For players without minor league options, it would be a little trickier -- you'd need to make it a IL/rehab thing, and the league may not like that. Or you could make it a minor league contract with a verbal promise to promote him soon, like Ervin signed with the White Sox this year -- although the league might not like a ~$15 mil "minor league contract"!
Late in Roger Clemens’ career I seem to recall him signing very late. Like into May late. I don’t recall what he did though as far as getting ready for the season.