While reading this mechasuit project log, I saw this:

This is the kind of thing I need to actuate the omniwheel. This would be used along these components:

Honestly, I only need 2 linear rails and one 300-500mm long stick, but for consistency (and because I can't find a 1pc listing) I'm going to use an empty 3rd HGR rail for the 2nd stage. The entire weight should be in the ballpark of 2.5kg, though the lengths required are subject to change since I don't yet know if it'll all fit in a 600mm tall backpack.

HGR15 has a cross section of 15*15mm, which should be enough to hold the weight of a packed backpack. There are likely cheaper ways of going about this, but the width of this needs to be as low as possible to prevent collisions with legs and maximise space available inside the backpack. I have more confidence in an otherwise unsupported HGR15 rail compared to an MGN12/15, with my concerns mainly being around the bearing static load.

For actually holding the actuator in the fully extended or fully closed state, I'm thinking of a servo connected to a small suspension spring, similar to the ones I found for #TEOSS [gd0037] a while back.

Oh. 450lb? Those are much too strong. 

Seems like gas springs are what I'm actually looking for.