Yes can use a text link to Google video or any other page for that matter, wouldn't be practical to not allow that. If it was practical I wouldn't allow that either...
I'm not trying to be p**** but I hate pop-ups and spam pages which is usually the types of pages that image and video services provide. Google and youtube aren't bad but a lot of the image services are. Files usually expire after a few months and some other image is being served up.
Addtionally there are other issues with hot linking, unless they are letting you some webmasters like myself do not like hot linkers. They suck up bandwidth and if it's on a very popular site it can be an enormous amount. I as the webmaster have ways of getting back, I can serve anything I want where the image request isn't coming from one of my pages.
Add to that the user never knows the differenc because the image is cached on their HDD, everyone else gets the replaced image and they usually don't find out until someoje tells them. These replaced images can be anything.....
As far as embedding google video or any other video here you can't, you can try if you want.
That would require either HTML to be enabled or compatible bb code neither of which is available on this forum. HTML is disabled for security reasons for both myself and more importantly the users. Do a search on XSS. Phpbb is hardened against such attacks and as of right now there is no known exploits but I don't want myself or the users to be the victims of possible future exploits.
Note that I have changed the file upload limits to 30MB for video, if you have any problems let me know. If you have a larger file than 30MB PM me and I'll set up a FTP upload for you which is unlimited.