I'm not sure Forrest would have been of much use if the United States intervened in the Ten Years War. He was pretty much used up, the war, Reconstruction and the little war in Tennessee waged against Governor Brownlow were probably ruinous to Forrest's health.
This wasn't too long before he became diabetic. If it had come to pass he no doubt would have given it his all, but I think he would have had to defer to his subordinates, and not ever been able to perform the herculean feats he was famous for during the CW. Especially when its remembered how small a role cavalry had in the Spanish-American War, which was probably bigger than what they would have been able to do in the 1870's.