How do I generate files and then zip/compress with Heroku?

I sort of want to do the reverse of this.

Instead of unzipping and adding the collection files to S3 I want to

On user's request:

1. generate a bunch of xml files
2. zip the xml files with some images (pre-existing images hosted on s3)
3. download zip

Does anybody know agood way of doing this? I think I could manage this no problem on a normal machine but Heroku complicates things somewhat in that it has a read-only filesystem.