Chagrin:
Cheaper options for linear actuators.If you're looking to jury-rig something take a look at a scissor jack for inspiration. The design keeps all of the weight off the gearmotor and avoids worrying about thrust bearings or such. For a dump mechanism it also has the advantage of a faster rate of extension near the end of its travel (when all the weight has been shifted to the platform's hinge).
That advantage is also a disadvantage, though - it has a hideous leverage in the lowered position which means you need a bigger motor or lower gearing which is then redundant in the higher positions. In general I think it'd be better to have a constant leverage.