I am going to offer a workaround for the issue of sending rich email with multiple captions. I don't know if this is the best solution, but it may work for you -- depending on what email client you use. The method I just tested used Thunderbird.
To email multiple images with a separate caption for each image,
1. select the images and then hit command-control-x (Export)
2. select "PM Classic" export template
3. You can ignore most of the settings in the template, but scroll down to the "Thumbnails" section. Make Max Width and Max Height something suitable for your ScribbleLive application. 1200x1200 is what I tried. (EDIT: Also you should change the number of columns to 1)
4. Add {caption} to the Title field for thumbnails
5. Click "Export" PM will ask you where to Export to. I suggest creating a new folder somewhere.
(Once you set these different options, they will be the default and you wont need to set them all up again. You could save it as a snapshot with the lightning bolt icon as well)
In many email clients, you can set an HTML file as "stationery" for your email. In Thunderbird this is in the upper right corner of the compose window. If you navigate to the folder you created in the Exporter, use the index.html file there as your stationery. The email then has all your photos with captions.
In my tests, this was actually pretty fast once i had everything set up. Every time I use the PM Classic exporter, I just overwrite the previous export. Thunderbird remembers the index.html as a choice in the stationery menu so i don't have to look for it every time. This worked great for me using Thunderbird, I don't know about other email clients.
Let me know if i can explain any better. Theoretically, you could customize and fine-tune this by creating templates using the HTML exporter as well. Then you would use the exported HTML from that as your email stationery.
http://wiki.camerabits.com/en/index.php/Web_Galleries_and_Export#HTML_Template_ExporterMick