As promised I wrote the patch, tested it, and it seems to work.
You can enter the max. amount of sql space available for resellers, who can make parts of their space available for webs under their account.
Statistics are shown both in the web's statistic tab and in the reseller's overview page.
German and english translations are included, other languages' strings are currently shown in english.
As the patch is to big to attach it here, I sent it to dev [at] ispconfig [dot] org.
I hope it's useful and will be soon integrated as it's been a lot of work
I tried to change as few as possible, and tried to keep the programming style of the changed files.
Btw, the patch was built against a fresh and clean unpacked version of ISPConfig 2.2.1