Comment 8 for bug 306356

Actually, no, it doesn't - base-installer is acquired by the installer at run-time.

For hardy and intrepid, pass apt-setup/proposed=true to the installer on the kernel command line.

For dapper it's more effort. I suspect the easiest way is to make sure that you aren't preseeding the locale, and then when the locale prompt appears, switch to Alt-F2 and edit line 113 of /usr/lib/debian-installer/retriever/net-retriever to add "$codename-proposed" after "$codename-security", so that the line looks like this:

                for codename_extra in "$codename" "$codename-updates" "$codename-security" "$codename-proposed"; do