It would look like a 'custom' folder in the effect presets. Or a dropdown list similar to the colour correction presets.
I've tried keeping a repo that I want to reuse in compound clips for copy / paste attributes - but the compound clip has to be pasted to the timeline / stepped in / copied / stepped out / deleted before I can then paste attributes - so that's not a very neat workaround.
I find it very hard to believe that there is no easy option for this other than paste attributes now this software has been around for 3.5 years.
I assume that with Motion 5 (I'm on the trial) I could do a repo and save as an effect preset.
'Make favorite motion' I use a lot in FCP7 - rename the resulting preset and reuse. So much goodness in that software that seems to be lost forever...