Yes this would be useful.
Most of the machinery for this is already in the DataFileWriter class. It is not exposed in a command-line tool though.
I currently use this machinery to take a large list of small avro files and merge them into one larger avro file with a set compression type and level.
In addition to the compression level, there is the concept of forcing a re-encode. By default, the current code will not re-encode unless required. Therefore, it won't re-encode deflate:1 to deflate:3 by default unless told to by passing in the flag to force it to re-encode. By default it will decode deflate to null or encode null to deflate. If a block is already compatible, it just copies the raw bytes of the block, which is very fast.
This tool should also support concatenation of files and creation of one larger file from a collection of smaller ones (of the same schema) with the requested encoding. Maybe something like this:
$ avro-tools append_to -f outfile.avro -c deflate:5 infile.avro [infile2.avro, . . .]
Which would create outfile.avro with codec deflate:5 form multiple source files.