Most process are currently re-niced by procwatch, so there’s really no need for that. You might want to check the Control Panel though and see if there’s a suggestion to add mysqldump to procwatch’s “whitelist”. I believe someone in IRC mentioned this yesterday, so you aren’t alone at least
Your other option though, would be to break the dumps up into smaller pieces. There might even be some programs available on the internet to do that for you.