Inserting said MMU into a Mac would require a piggyback board housing both the CPU and MMU; the 68451 has to straddle the address lines (and a bunch of control signals) between the CPU and main memory and, somewhat infamously, drastically changes the RAM timing. (The 68451 adds a wait state and it also apparently wasn't really designed very well for demand paging as implemented in "modern" UNIX variants; the limitations of the unit were severe enough that many vendors, most famously SUN, just designed their own more efficient MMUs.) Arguably you don't gain that much over just building your own single board computer trying to upgrade a Plus with an MMU.
(Perhaps more mundanely, you:
A: Don't need an MMU for uClinux anyway, and,
B: I'm *pretty* sure there's no "full" Linux port that works with either the 68010 or the 68451 so it's not as if wedging an MMU in there is suddenly going to give you more software to run. The only free UNIX-oid I know of that works on any 68010 computer is the NetBSD port for the Sun2, which, again, uses the proprietary SUN MMU. It might well be possible to duplicate the SUN MMU with a few CPLDs and a fast static RAM chip; here's the engineering manual for the CPU board, it doesn't use that many parts.
http://bitsavers.trailing-edge.com/pdf/sun/sun2/800-1185-01_2-120_CPU_Engr_Sep84.pdf
That might be a really educational thing to do on a single-board computer... but certainly not a trivial project either.)