Not strictly a Dolphin modification, as it applies to TinyMCE in general, but I will outline it here for Dolphin 7. It's a pretty easy modification to do, just two files to edit.
The functionality for this is already in TinyMCE, but for some reason it is not enabled. I've tested this out on version 7.0.2 with no issues.
To enable YouTube /flash video button in the forums
Find the following line
theme_advanced_buttons3_add : "emotions,iespell,flash,separator,print",
Change it to...
theme_advanced_buttons3_add : "emotions,iespell,flash,separator,print,separator,media",
Now compile the forums, and you should see a new button
To enable YouTube /flash video button for the rest of the site
- Copy the url from the video's page,
- Click on the button in TinyMCE.
- Select 'flash'
- Paste URL into URL box
- Click insert