Here's how I got this to work (at least as far as MPEG Streamclip is concerned...)
Even if the installer wont work in Lion, the component inside works fine, the only problem is to how to get it out of the installer:
1. In Finder, right-click on the installer package QuickTimeMPEG2.pkg and select "Show package contents"
2. Inside the package there is a directory called Contents and inside that a file called Archive.pax.gz
3. Copy Archive.pax.gz to a temporary directory on your Desktop, for example.
4. Extract the contents of the archive by double clicking it
5. After extraction, you should have a directory called System next to the archive.
6. Now just dig into the directory to find the component file QuickTimeMPEG2.component
7. Copy the component file to the correct place (/Library/Quicktime/) and Bob's your uncle.