I think this is the exception, not the rule, but there is a Friesian x Arab cross (Golden Knight) that completed Tevis this year. Which is extra impressive, considering the extreme heat at this year's Tevis, it would be extra hard for a non-Arab to complete.
I don't think most Friesian crosses would be suitable for endurance though - it would have to come out just right without being too heavy.
However, I think most sound horses of any breed are capable of completing LDs with the right conditioning... so it really depends what your goals are, and how your baby turns out.
As someone who is attempting endurance on Paints, I say go for it! Enjoy the ride, take it easy, listen to your horse, and don't plan on being competitive or racing. Condition carefully, ride conservatively, and just see where it takes you.