I think I can answer my own question.  I've managed to get this to work using the maple-graded question type, formula answer, symbolic entry only (so that prime notation can be used).  The key seems to be that the maple-generated answer contain the operator D rather than the command diff.  For example, if you ask maple to differentiate sin(f(t)), maple will respond with cos(f(t))*diff(f(t),t).  But if you ask maple to differentiate sin(f(c*t)), then it responds with cos(f(c*t))*D(f)(c*t)*c.  You just need to substitute c=1 into the result. 

The following algorithm works for a question like "find the derivative of sin(f(t)), assuming f is differentiable":

$g=maple("sin(f(c*t))");

$ans=maple("subs(c=1,diff($g,t))");

This will enable students to enter their answer symbolically using f'(t).