To be honest, I HATE those velocity controlled slides. A guitarist can play a hammer-on VERY quietly, and play a straight note very loudly, but these patches force you to ONLY play hammer-ons at their loudest. A poor compromise, IMO... I would much rather they be triggered by a footswitch (just before the note). Then they remain independent of the velocity.

But, until then... for those of you having trouble triggering them from 88's, try looking for an 88 controller that you can customize the velocity response on a per-setup basis. Some of them allow you to tailor the response very specifically. OTOH, if you are dealing with Korg's (don't know about 500's, but 800's can do this), those trigger points for alternate samples are editable, and you could tailor the voice to suit the 88...

Your choice.