Could you provide the link to example, please.
In general, I think it is not a convenient to use a modal element in this particular case. It should be something like a showToolTip component. Modal Panel is a component that presume you will do something with internal content (editing input fields, pressing button) while the rest content on the page is not available. Hovering over the row is not such case.
This modal panel will have some editing fields and a submit and cancel buttons. I've seen it done at www.gmail.com under Quick Contacts list. (off course you need a gmail account :) sorry.
Ok. I see. It is not a modal element there. rich:modalPanel is not designed for such behavior. It is a good use case for new component.
In general, I think, it is possible to reuse the modal panel with extra steps.
In any case, is better to have only one panel instead of one panel per record. You need to calculate the position for panel before it is shown. You need to close it on on blur.