To play around with a non-technical solution I would consider rushing out a "Version 2.0" of the application that accomplishes the following (if possible within a <24 hour window):

1) Has a functioning update service that fixes the original problem

2) A quick re-skin/theme-change to i.e. a more "modern" version of its original design

3) Whatever performance/security improvements you can cobble together quickly or were in the pipe already

I would do this because I see this as also a business/finance/sales/marketing problem as a technical one. Assuming this bricked the core product, you have to save the business first, then save the quarter, then the week, then the day.

I think this would work because it is much easier to get a user to re-download a version 2.0 with performance/security/design improvements then it is to download almost anything else (imo). Again, this is assuming no saner solution exists. Interested to hear your accepted answer though.