The National Probate Register here is for the UK where there was no slavery. But perhaps a master might move to England or Wales bringing slave with them and then leave them a bequest.

Your question needs an expert, but my guess (which you probably already know, my apologies if you do) is that to find a slave named WITHIN a will you would need to know or guess the name of the master making the will? Presumably slave owners did leave bequests to their slaves ... including the gift of freedom?