Equation-based amplitude is definitely missing. But creating moving loads would require the equation to be position-dependent (x, y, z) and time-dependent (t). Then, PrePoMax would generate an individual amplitude for each node. But it would only work for nodal loads and BCs.
Abaqus doesn’t really have a feature that would allow easy modeling of moving loads. One has to use workarounds with amplitudes, subroutines, scripting or even contact with rigid body moving along the structure. As a side note, there is the BRIGADE software which uses Abaqus as a solver, but is meant for bridge design so its features include moving loads, load combinations and so on.