The on-disk OpenEXR file format may be the source of another objection:
In order to support random access to scan lines and tiles, a scan line or
tile position table is inserted near the beginning of the file. With
lossless compression the position of the scan lines or tiles within the
file depends on image content, and the table entries are not known until
the corresponding scan lines or tiles have been stored in the file. Once
all the pixels have been saved in the file, the OpenEXR library must perform
a seek operation back to the beginning of the table and write the table
entries. If the files are recorded on a streaming medium, for example, a
tape, a backwards seek operation is incompatible with real-time performance.
This means that the entire file would have to be constructed in an in-memory
buffer before it is appended to the output stream. However, for B44-
compressed and uncompressed files, the scan line or tile positions are
known in advance because they do not depend on image content. Seek
operations are not necessary, an the file could be streamed out to the
recording medium while it is being constructed.

Overall I think it should be possible to use OpenEXR as a capture format,
but as I mentioned above, there may be problems that I don't know about.

Florian