Yeah there are a few ways to do it. Just be sure not to upgrade your firmware ever. If you do, you'll never be able to install Rejuvenate and 3.20 should be fine for just about anything (except for really new games that require higher firmwares).
One way is if you have a PS3 on the latest OFW you can use that to visit the PSN store and download the things you want, then you can send the files to your PS TV over LAN (this also requires you to run Vita Update Blocker, otherwise it will force you to upgrade your firmware).
Another is to download the things on an up-to-date Vita, then move the memory card to your PS TV. However, before taking it out of the Vita you need to use the EML hack to unlock the memory card so that it won't force you to update as you plug it into the other device with the lower firmware version.