OK - I just looked through all EMD 645E3 sound projects for Select decoders (there are quite a few, not just the file made for IMRC) and, unfortunately, none of them have a separate horn in another sound slot. While this would be very easy to do in a full V4.0 decoder (custom sound files), only ESU can make Select sound projects.
The horn, in Sound Slot 2, is controlled by the horn selector bits in CV48. There are potentially 16 distinct horns available in any given sound project. But one cannot switch between them "on the fly" (would be a neat feature). Some projects do have alternate horns (usually electronic sounding ones, or special grade crossing sequences). Most projects have a number of alternate bells
If a Select sound project did have an alternate horn, programming it to be the horn for reverse direction would be simple. In the Function Mapping tab (of either LokProgrammer or DecoderPro), there is a row which shows "F2" under the "Conditions" column; and either the name of the primary horn (in LP) or "sound slot 2" (in DP) under "Sounds" column. What you would do is change the value in the "Conditions" column for this row to show "Forward, F2". Then, at the bottom of the function map, you would add a new row (the order of the rows in the map doesn't matter), with "Conditions" = "Reverse, F2". Then, under "Sounds", select the alternate horn sound slot.