Linking invoices to bills / bills to invoices

Linking invoices to bills / bills to invoices

Is there any way to do this ?
Is it possible through the api ?
I've given it some thought but can't seem come up with anything !

I was thinking of using notes to place a HREF in, but no joy ;-(

I think that if you guys provided a HREF choice in custom fields, it would be totally invaluable....
I magine the possibilities  :-)