Dependent parameters, keyframing.

I'm looking for some help creating dependent parameters that interact correctly with keyframing/expressions/scrubbing.

As an example imagine I have 2 float sliders: SLIDER1 and SLIDER2.  I want my effect to copy the value of  slider1 into slider2.  I've discovered the PF_ParamFlag_SUPERVISE flag, so when the user adjusts slider1 I can copy the value into slider2 during the PF_Cmd_USER_CHANGED_PARAM callback:

    params[SLIDER2]->u.fs_d.value = params[SLIDER1]->u.fs_d.value;

    params[SLIDER2]->uu.change_flags |= PF_ChangeFlag_CHANGED_VALUE;

That works but fails when SLIDER1 has been keyframed (or uses an expression).  If I add 2 keyframes on SLIDER1 and then just scrub the timeline then PF_Cmd_USER_CHANGED_PARAM is never called.  The supervisor sample just seems to use a non time-varying supervised parameter so that's not so helpful.

Are there any callbacks/flags/idioms/hacks to create the effect I want?   It's not ideal but I'm thinking about baking keyframes in SLIDER2 whenever SLIDER1 is changed :-(

Thanks!