Not as far as I am aware anyway...
The only way I have found to do such a thing (and it was quite a while since I tried) was more of a hack than anything, it required a script trigger plugin and was not effective enough to use for a deployed solution, but it did kind of work.
What I did was to use the tooltip of an object to trigger a script, passing the object name as a script parameter.
The script would then set a variable and refresh the screen.
The conditional formatting of the object would look at the variable to decide whether to highlite or not.
However... this poses a problem when moving away from that button or field, there is no way to un-highlite it without having the rest of your layout clear the variable using a similar method...
Of course, if somebody does have a technique which works reliably, then Im interested too....
FM doesn't support mouse-over events natively. When you think about it that seems strange because when FM displays a tooltip it's in effect performing a mouse-over event. It doesn't seem like a stretch to allow the developer access to the function. I like SWS's work around . . . if only there was an equally graceful way of un-formatting after the hand or cursor leaves the object. As well, tooltips seem to take time to activate . . . at least the first one. Perhaps that's adjustable and I've never noticed.
I know this is kind of 'off topic' here, but I thought I would mention it anyway...
I use a similar approach to activate a portal, when moving the mouse over it...On a layout with multiple portals, it becomes a pain to have to click in each one (and miss a field) before the mouse wheel works properly.
Using the tooltip & a script trigger plugin, I basically trigger a script to go to the portal object in question, meaning each portal become active just by hovering over it... There is a small delay on the first one (like you say) but overall, its what my users wanted... so its what they got.