The Boot Panel in Tiger is now embedded within the WaitingForLoginWindow program in /usr/libexec/. Here's a quick AppleScript applet to replace that PDF (not for Server version!). Just change the output path then drop a PDF on it. The PDF has to be exactly 12517 bytes, so quite small. However, the script will pad the PDF with whitespace to increase it to that size if it's smaller.
Also, here's a perl script to test if it worked; just pass the filename to it. And finally, a quick installer that requires sudo/root access.
[robg adds: I haven't tested this hint -- I spent some time trying to make a small-enough PDF, but then gave up! In pre-10.4, releases, there was simply a PDF which could be released (see this much older hint).]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050517121748796