When you "multiclass", you're still only your original class.
Unlike a hybrid, which counts as both of its hybrid classes for all purposes, someone who's taken a class-specific multiclass feat only counts as that class for certain prerequisites. For example, consider Baron Geoff, a Warlord who's taken Student of the Sword, a Fighter multiclass feat, and Baron Theo, a Warlord|Fighter hybrid character.
Both Geoff and Theo can take feats that have Fighter as a prerequisite, and both Geoff and Theo can select paragon paths that have Fighter as a prerequisite.
However, when Geoff and Theo reach level 7 and can then select an encounter attack power "from their class", Geoff can still only select from the Warlord encounter attack powers, while Theo can select either a Warlord or a Fighter encounter attack power.
As a GM, I'd be given to a generous reading of Reserve Maneuver "in the spirit of" multiclass counting for meeting feat prerequisites. Ultimately, though, Reserve Maneuver is not Reserve Maneuver (Fighter) that gives you a Fighter encounter attack power, prerequisite: Fighter, but is a single feat that only refers to "your class", and even though it's called "multiclass", you only have a single one of those.
By that strict reading, then, Geoff can only pick a Warlord encounter attack power with Reserve Maneuver, while Theo can pick either a Warlord or a Fighter encounter attack power.