Since it seems Shadowmourne will be available from 25-man normal modes, do you think there is any chance Blizzard would make the weapon available for the 10-man Heroic version of ICC? Historically, the two have always awarded the same iLvl of gear, typically even with some BiS pieces in the 10-man Heroic version.
I've been monitoring some of the posts on the official forums on the subject, and one of the major concerns seems to be that 25-man guilds would farm 10-mans to get the legendary faster, but I really don't think that would be a problem. There's only 3 things that need to be farmed in the quest chain: Primordial Saronite (which doesn't bind, so you could just buy these off the AH if you needed them), souls (which can already be obtained from either 10 or 25), and Shadowfrost Shards (which is pretty much the only real problem).
The Shadowfrost Shard problem could be easily fixed by splitting the quest chain in 2 at that spot, giving a "The Splintered Throne (Heroic 10 Player)" quest and a "The Splintered Throne (25 Player)" quest. The 25 player version would be exactly the same as the one that exists now, and the Heroic 10-man version would simply have you collecting similar shards (same name and icon, but different ID) from 10-man Heroic bosses. You'd only be able to choose one of the quests, and both would lead back into the same spot of the questline.
This would allow both Heroic 10-man groups and 25-man groups to obtain Shadowmourne without forcing 25-man groups to do any more 10-mans than they'd already need to do.