I heard that Armstrong was at the Winnipeg-Philly game today. It might have just been a convenient game to take in en route to scouting something more likely, but it's something to consider.
I think Philly is a no-go. They've turned it around lately and, while they have the center depth that the Blues covet, they need puck-moving D, which is something the Blues cannot spare. A deal might be possible there, but it seems a lot less likely than one with the Jets.
Players the Blues are probably looking at on the Jets: Hainsey, Little, and Burmistrov. The Jets have also been heavily rumored to be in on the O'Rielly circus. The Avalanche need puck moving D, but they were also rumored to be asking for a package of a higher pick and a young forward.
So...
The Blues deal:
2013 1st rounder
A significant prospect
Russell
D'Agostini
-for-
Hainsey
Burmistrov
2013 pick (3rd rounder, possibly Chicago's 2nd rounder)
Then Winnipeg deals the Blues' first rounder and a young guy (Postma, Klingberg, Sheifele, whatever - anybody but Trouba, probably) for O'Reilly and promptly sign him - possibly for less than he's demanding elsewhere because he'll immediately step in as their #1 center.
I honestly don't see this as the Blues unloading trash. A first and a significant prospect is the core of the deal. The Blues want to stock up on 2nds and 3rds in a deep draft, so if I'm the Blues, I want to weasel that returning pick into a 2nd rounder. If I'm Winnipeg, I want Jaskin or Binnington and I want to talk that 2nd rounder down to a 3rd rounder. I think they'd honestly want D'Agostini. They need right wingers and they need depth. I have to believe Hainsey would waive his NTC. They're willing to move Burmistrov in order to get O'Reilly without giving up their own first rounder and get higher quality return on Hainsey. The Blues can put Burmistrov in as their 3rd line center, move Steen back to wing, and Burmistrov and Tarasenko could work well together when Tarasenko is healthy.
Jets seriously improve their scoring lines and get a solid return on Hainsey, the Blues get better, and the Avalanche get well-paid for O'Reilly. The reason the Blues don't make the deal for O'Reilly straight up is that his contract demands are too rich for them and they want a defenseman, too.