As far as Shinto Muso Ryu jodo goes, I've heard the same story about Gonnosuke Muso and his "invention" of the jo -- going off to the mountains for 37 days (if I remember correctly) and coming down with his "new" weapon to defeat Musashi.
Other stories about the jo is that it's just a walking stick ("jo" has the same kanji character as "tsue" which means "cane" in Japanese) or that it's a form of "back-up" combat for spearmen who has their bladed tip of the spear broken off.
I don't think the founder had any real formal studies with the jo nor the spear... Stan Pranin in his interview
on this site says about the founder:
Well, take the yari for example. He received some juken (bayonet) training in the army, but so did I! I'm sure he did a lot more than I did, but in that context you're not doing a martial arts type of training. The yari was probably an extension of that bayonet training and whatever else he learned along the way. We know he did a lot of self-training during his Ayabe years at the Omoto. There are anecdotal evidence that he would use a yari in his practice, but there is no record of him having formal training.