I would be highly surprised if mails (even spam) wouldn't be scanned for steganographically hidden messages (no insider knowledge though). Hell, I would even want this for my own antivirus functionality.<p>I would be surprised if your stegospam wouldn't have a red flag raised by any scanner which utilizes advanced heuristics. Especially since your stegospam utilizes a form of steganography which is already described into great detail by e.g. David Kahn in The Code Breakers (nice crypto history book).
After stegospam pass, have you not some html redundancy free room to utilize?<p>Are you aware of deogol, perl, stego?<p>Not much gained, but there is a scanner how much free space per html file size.<p>..........................<p>Deogol<p>an HTML steganography tool<p>What is Deogol?<p>Deogol is a commandline Perl program implementing basic steganography on HTML files<p><a href="http://hord.ca/projects/deogol/" rel="nofollow">http://hord.ca/projects/deogol/</a>