I think such relationships prove the general soundness of Dr. Friday's thesis. You have Iizasa Choisai reportedly teaching Shogun Ashikaga Yoshimasa, Tsukahara Bokuden teaching Ashikaga Yoshiteru, and when Kamiizumi Nobutsuna went to Kyoto in the mid-1500s, he was teaching the high-ranked "nobility" of the Shogun's court. Ieyasu's keppan to Yagyu Munetoshi mentioned nothing about teaching his troops or even his family, but was a personal oath to learn Shinkage-ryu. None of these men were expected to "lead from the front", so the practical utility of learning these traditions as fighting arts was not very significant. Rather they indicate that the daimyo and shogun saw these arts as being philosophically relevant, in the teaching of strategy, tactics, self-improvement and human interaction. Yagyu Munenori's relationships with Hidetada and Iemitsu were very much in this vein -- Munenori's value to them was not in his strength of arms, but as an advisor and teacher. While it's tempting to classify this as being part of the changes peace brought to the Edo Period, I think it's important to remember that this is a historical distinction, looking back from our modern day. To Hidetada, Iemitsu, and Munenori, they were not in the "Edo period", or the "Pax Tokugawa". Battles were still in living memory, and all three of the above were veterans of battle (Sekigahara and the Siege of Osaka Castle). To them, their time in history was not particularly different from that of their long-warring fathers and grandfathers. And yet, the relationship that formed was not particularly focused on the nitty-gritty of using Shinkage-ryu on the battlefield.
Josh Reyer
Swa sceal man don, žonne he ęt guše gengan ženceš longsumne lof, na ymb his lif cearaš. - The Beowulf Poet