If you mean "if some device tries to tell the RFC1918 box the packet is too
large", this is correct. However, the same reasoning shows that *ANY*
session opened between an RFC1918 box and a random host on the Internet
will break as well.
I suppose you could be talking about a one-way transmission of data from
the RFC1918 box to a random host. If so, yes, you are SOL. I just don't
see a whole lot of that. 
TTFN,
patrick
I Am Not An Isp
www.ianai.net
"Think of it as evolution in action." - Niven & Pournelle