Is there a way to conditionally display a tab in a workitem?

This has been answered several times in this forum and I think the general answer is no.

For all I know, it is not possible to conditionally hide or show a tab.
It is possible, however, to hide a field by default and to show it only if a particular link type is present on the work item.
This is an out-of-the-box feature of RTC, just have a look at the field presentation properties.
So, if you want to show a field only if another field has a specific value, you could implement a follow-up action that adds such a link (to a generic target) to the work item when the other field is modified and gets that value.
Actually, I recently implemented such a solution, and it works fine.
The RTC version was 6.0.3, but it should work with 5.0.x as well.