Garner and Chaudhri's book 'The Propagation of Fruit Trees' has almost nothing on using cashew rootstock for mango. Only this, on page 421:
"In Martinique, cashew seedlings have been reported to be compatible as rootstocks for mango, and the fruits on cashew roots was almost double the normal size, free from fibre, with a smaller seed."
Sounds too good to be true. I wonder about taste.