I thought that those keyhole bits had big labels on them basically saying “Don’t use in a CNC machine!” They’re really designed for either a plunge router or a router table where that over-constrained bit has the freedom to shimmy the router or wood around if it needs to. If you have decent hold downs, and that bit decides to stick or bind or whatever, something’s going to give, and likely violently.
Plus, chip clearing is less than ideal once you get past the initial plunge. You’re hogging out more material than the slot is wide. You’d probably want to have a tool change with a straight bit making a clean slot first, then going back to get the wider slot afterwards.
EDIT: It appears that some keyhole bits are made for CNC machines. But they all seem to have 1/2" shanks (or larger), and my guess is they are designed for industrial-class machines, not hobbyist machines. To be clear, if you really want to use one in your MPCNC, it’s your shop. I would just warn you to use extra caution, and keep your hand near the eStop/circuit breaker/first aid kit… 