First of all, what ILS are you using? It may be possible to adjust the ILS driver so that it displays the appropriate temporary location at the top of the holdings tab (at present, some, if not all, drivers always display the permanent location -- I know that, for Voyager at least, a fix for this is being worked on).
The "On Reserve - Ask at Circulation Desk" message is coming from the template file in /web/interface/themes/[your theme]/Record/view-holdings.tpl. Perhaps if you can get the temporary location to show up correctly, you can simply change this message to something more generic or appropriate, and that will serve your purpose. Alternatively, you might want to make further adjustments to the ILS driver to pass back an additional field beyond the current simple "yes or no" reserve setting.
Hopefully this is some help, but if you need more information (like a more detailed explanation of what ILS drivers are and how they work), please let me know. Also feel free to make use of the vufind-tech mailing list if you plan on making code changes and have questions about the gory details.