Interesting suggestion about the spot meter - only have a Sekonic L-308S at the moment - the Lumidisc attachment might work pressed up against the eyepiece...
The Mode B live mode does have a time-out and it's certainly less than 8 minutes by default.
Interestingly, I exclusively used the live preview for the DXO test target tests I did today and I preferred Mode B because it gives a perfect fit view for 100%framing. The zoomed manual focus is also nice in Mode B.
It did make me wonder at the time if over-use of Mode B would affect the noise test results and I do plan to repeat the tests using Mode A and Mode B separately to see if there is a difference.
One annoying thing I've found is the one-touch custom white balance calibration procedure - it's impossible to fathom without the manual and involves pressing the release mode selector button at the same time as the shutter release. The E-500 has a dedicated button for this.
Ian