Assume I'm going for a pure single-classed build, and I don't mind devoting feats or other character options towards this--just not levels. Is there any way for a member of a given Initiator class, let's say Warblade as an example, to either gain access to or substitute one of their Disciples for a Discipline outside their class, let's say Shadow Hand?
There is not. Impossible to prove a negative, but Tome of Battle is really the only book that discusses maneuvers, and it’s not in there.
However, it is a really common houserule to allow a character to swap disciplines. Since the disciplines are fairly-tightly balanced, a one-for-one swap is usually fair and fine. Shadow Hand is arguably one of the “best” disciplines, while Stone Dragon is one of the “worst,” so maybe if you wanted to swap Stone Dragon for Shadow Hand that might be a little dubious, but it’s still pretty minor improvement in power. You’d miss out on mountain hammer, too, which would really be disappointing.
For that matter, with your limited maneuvers known, even gaining entirely-new disciplines doesn’t improve your capability all that much. If given the choice between spending a feat on a new discipline, or swapping a discipline for free, I’d go for the swap every time, for example. You could give all initiators access to all disciplines and it wouldn’t affect very much.
The real reason for the discipline lists is flavor. Devoted Spirit is unique to crusaders because it’s the “divine” discipline. Desert Wind and Shadow Hand are unique to swordsages because they’re the “mystical” disciplines. Setting Sun is unique to swordsages since it’s the “use their force against them” martial art. Iron Heart is unique to warblades because it is the “By CROM!!” discipline. They fit the classes they’re attached to—and they reinforce that class’s fluff by being unique. Part of the warblade’s schtick is being a mundane warrior, and that’s bolstered by the fact that none of their maneuvers are supernatural (even the ones that really seem like they should be, like earthstrike quake).
But this isn’t really that important. PCs are usually unusual, after all. For many tables, allowing a PC to be a warblade with access to unusual disciplines isn’t that big a deal.
Failing that, you can get a bunch of Shadow Hand maneuvers just by taking Martial Study. As a warblade, they’ll be added as warblade maneuvers, even though they aren’t on usually available to warblades. If you take it three times and know three Shadow Hand maneuvers, that could be half-or-more of your maneuvers readied until 20th level.