The true IDs for elements include all three parts - item type (REQ), package prefix (BO) and an actual ID number. As you mentioned, we already support pulling out part of that ID using $IDPrefix, though pulling out other individual parts isn't currently available. Since we already do one of the three, I don't think getting the other parts separated should be too difficult, and is probably a good idea.
We have come up with a couple of other workarounds. Though fairly clunky and time consuming, I think one or the other will enable you to achieve the results you're looking for.
The first suggestion is to use some sort of string function in an Excel report template that would strip out everything but the number in the IDs. You could then combine this with $IDPrefix to get the results you desire. I must admit I'm wholly unfamiliar with this process, so I'm not even sure if this is something that would work. Of course, this wouldn't apply if you're generating Word reports.
The other option is to use a custom field. Create a custom field for your items - you could call it IDNumber - then use that with $IDPrefix in your reporting (either Word or Excel) to generate a seemingly singular ID that uses the prefix and numerical identifier. You'd need to manually enter the ID numbers into the custom field, and of course manually update that if you ever renumber your items and change them.
I'm sorry we don't have a better workaround for you at this time. Thank you for the suggestion - we're always looking for ways to improve upon the software!