Forum managers don't care abour your directory structure or innocuous-looking web pages on the domain, they primarily ban IPs, no way to get loads of those unless with alot of running around free hosts (and abusing their TOS because they don't want proxies) or waste a sizeable sum in paid hosting/IPs.
Hosting providers can just close you for overuse of shared resources whether they find your proxy or not, because that's the main reason they ban proxies, not some unfounded hate out of the blue.
Also, most hosts worth their salt and of some size that ban the usual stuff (proxies, phishing, VBulletin, IPB...), have their accounts checked not through human visual browsing of pages but through automated crawling of files present on the server itself.
A wholly homebrewn script with innocuous file names may escape notice if resource usage is kept low but that's about it.