I don't think there is any direct/built-in way to do it.
I've noticed the list of file types supported by the picture method of lr-views is absent - do you know whether it works for jpg as well as png?
Its still possible you could use it though if you are willing to do a bit of work, for example:
- Call exiftool to extract a jpg from raws then display those.
- Find a way to tap into the lr preview subsystem and use the jpegs there.
Another idea: Make it a publish service instead of an export - since thumbnails for photos to export are already presented...
After doing some more investigation, I found that it may be possible to parse image thumbnails out of the lrprev files. LRViewer is an app that does this, as well as Jeffrey Friedl's 'Preview Extraction' plugin.
Unfortunately, for neither is any source code available, and Jeffrey's plugin also doesn't work in LR3 anymore. In his screenshot he is clearly displaying catalog images in a plugin dialog though.
I wonder why his plugin does not work in LR3 though... did the lrprev file format change a bit from v2 to v3 and he hasn't bothered to reverse engineer it again, or has the SDK become more restrictive, preventing these types of tricks in LR3?
Also had a look at your Lightroom Plugin Development project... very interesting. If I figure out how to extract a thumbnail from these lrprev files perhaps I could contribute it back to this framework...
Definitely possible to parse the images from the previews, or at least it was in Lr2. I would guess Jeffrey Friedl just never got around to updating the plugin, since he got sortof "plugin overload/burnout..." (he's got dozens of plugins that he makes very little money from... - its a labor of love whose novelty can wear off...
But I'm just guessing.
I would guess one could reverse engineer the preview subsystem fairly easily simply by locating the jpeg images in the files, and displaying them, rearranging them, dumping the data for them... I've actully loaded the jpegs from the previews directly in ACDSee before - dont remember details... So, the hardest part would probably be figuring out how to map photo catalog info to preview item... Unless of course Adobe has obfuscated them now so we dont "play" with them...
Does the LrViewer work with Lr3 catalogs?
PS - The SDK itself has not become more restrictive, except in a few insignificant cases, like finally removing setfenv...
Hope you mastermind it then fold it into the Lightroom Plugin Development project :-)
There's supposedly plenty of room for participation by others in the project although so far its been mostly me.