On a mast with the sail hooked at the top, the halyard line is unloaded during sailing...so the main luff is the only force making compression on the mast...and most of that is from the downhaul force.

Now if you run a base cleat instead of the top hook, now you've got two forces acting to make mast compression...the loaded halyard and still the sail luff. Input force is still primarily the the downhaul, but you get 2:1 purchase (halyard + luff) compressing the mast.

The extrusion can easily take this, assuming you've got reasonable diamonds and don't get crazy with >16:1 downhaul purchase. Biggest concern will be the top sheave & it's mount pin...not usually designed for these kind of loads.

The base cleat needs to handle at least as much force as you can put on the downhaul line..plus some margin for gusts, etc.

Mike.