The same happens when trying to import it. If I change the extension to .img then I can mount it just fine in the Finder so I don't think it's corrupted. I tried different IMA files from different sources.
If you rename the IMA to IMG and Finder will mount it, then it's definitely not a DOSBox-compatible IMA file. Finder can only mount IMGs if they are in an ancient OS X-specific image format; not the Bochs IMA/IMG format supported by DOSBox.
I need an example IMA file in order to diagnose the problem properly. Boxer's IMA image support has been tested to work with IMA/IMGs that are compatible with DOSBox. If IMA doesn't work in DOSBox, then this isn't a Boxer bug.
OK, the provided IMA file (now attached to this issue) works in Boxer 1.1 for me, and can also be mounted by Finder - not sure why the latter is the case, but anyway. This could very well be a PowerPC bug, i.e. there's some endianness problem. Could you Force Quit Boxer while it's hanging, and post the crash report as a comment on this issue?
Yup, there is indeed an endianness bug in DOSBox, when reading the image file's header into a struct. This neglects to convert certain header data from the little-endian storage format into big-endian integers on PowerPCs and as a result, later read operations will try to seek beyond the end of the image file and die a horrible death.
...on closer inspection, directory reads (and writes) from FAT images are also riddled with endianness bugs. And these haven't been fixed in trunk either. It's a wonder nobody's reported it, but then FAT images aren't very common in the wild.