Water Blast, Street Justice, etc, is what I was thinking for using as bases for kludging the i24 Snipe changes. It might even obviate the need for ADDING powers, as you could just repurpose ComboLevel1 .. ComboLevel3:
Aim/BU give you ComboLevel3
Targetting Drone gives you ComboLevel2, or 3 if you have a PowerBoosted buff (Power Boost does not add a marker, but we can repurpose another power to use for it).
Tactics gives you ComboLevel1, or 2 if you have PowerBoosted (this will require some other checks, because we don't want it to help a melee set that actually uses ComboLevel).
Overgrowth gives you ComboLevel1, or 2 if you have PowerBoosted.
Kismet IO gives you ComboLevel1
All levels are cumulative.
It looks like it should work to mimic the mechanism reasonably well, with the change that Tactics is equally useful for FastSnipe for all ATs. Which, frankly, is to my preference... I did not think that Defender blast sets needed to get an easier FastSnipe than the equivalent Blaster sets.
EDIT: After thinking about it, you would run into too many problems with outside buff powers such as Fortitude or Overgrowth giving free ComboLevels to Scrappers/Stalkers with the right powersets, which you do not want. However, a version that avoid this problem would use NPC buffs such as the buff powers that Lord Recluse gets in the STF... set the powers so that their buffs work specifically only on Lord Recluse, and then you can just grant them upon any other PC or mobile as "flag" powers like the ComboLevel powers, but without running any interference with non-Snipe powersets.