Well, something I don't fully get here, you mean that you have issues because some URL are too long ?
What happen, are they as is seems not working because of their length ?
And this only concerns adds urls ?
In all cases, it seems to me that if you have too long urls it means that we'are talking about several dozen of chars, and if so saving ten chars is not like the final solution, as it would not bring you this far from the limit.
About the redirect, if you mean that you could use another domain for these too long add urls, and redirect it then it would not change much, since the add url actually loaded would in the end using the target domain, eg the long one if I get things well.
A solution would be to fully switch to a shorter domain in such case, keepeing the old one there only to be redirected to the new one. Would be like if you'd change all your site url, does not cause trouble SEO wise as long as the redirecting is in deed working.