MapCompare, as most pages, uses our default tooltips. Adding it to MapCompare alone makes no sense. It's not likely that most will want it there but not in other places. Adding it everywhere would start to bloat them (then someone else wants another string and so on).
What is planned, and has been planned for a long time, is a way for paying members to customize tooltips. Either per page or globally. This is not a high priority task, but the idea exists in our TODO-database.