I am pretty sure, that the mast of most beach cats can handle the doubled load. But on some lightweight constructions (e.g. Acat) I would be careful.

However the luff tension acts not only as compression force on the mast, it bends it backwards as well, an effect which is normally desired. With a conventional halyard you will roughly double the bending for the same amount of downhaul tension, hence the leach will open more than usual, or with the same twist (leach opening), you will have less downhaul and with that a fuller sail. But maybe this aspect is of a lower importance if you don't sail races.

Cheers,

Klaus