I suspect it the rule has to do (hah!) with the adjective rule as well as the desire to avoid confusion with some sort of verbing. I think the nouning of it is typical English language vocabulary creativity.
One factor in being consistent, of course, is to assist translation.
I never saw HOWTO until I was looking up something on a Linux CD one time. I had seen man pages for years before, of course.
Enough speculation!
Your mention of "style guide" reminded me that there is a Microsoft Manual of Style for Technical Publications, though I don't know if it has been kept up. I had to look at three wrong choices before I found it in my disorganized bookshelves. It actually has "how-to" in the index.
Here is the Entry:
"how-to vs. how to
" Hyphenate as an adjective (as in
'how-to books'), but use two words
as an adverb plus infinitive (as
in 'This is how to format your disk').
" In headings and titles, do not
capitalize the 't', as in 'How-tos,
Tips, and Tricks' or 'How to Format
Your Disk.' "
Notice that they don't address nouning, but they use "How-tos". (The so-called computer dictionary is worse. Fortunately or unfortunately, the dictionary has no entries with title in the how* range. Clearly Bill Gates did not proof-read any of this or some serious howlers would have been eliminated. Bill is very astute concerning language and clarity.)
Citation:
Microsoft Corporation. Microsoft Manual of Style for Technical Publications, 2nd edition. Microsoft Press (Redmond WA: 1998). ISBN 1-57231-890-2 pbk with CD-ROM.
Experimentation:
I thought I might try searching the CD-ROM version of the document. There is a Microsoft .chm version, but it's search treats "how-to" as two separate words. Bummer. Ahah, they talk about "readme files" not "read-me files" but that is probably a typographical issue, since the name of the file is being referenced. I suspect that is the ONLY case where "howto" is usable, as well as fully capitalized, when it is about files having a particular name or the application that coughs up such files (the howto command).
Laughs:
I tried running the install of the book on the CD-ROM using Windows 7 64-bit :). The installer said it needed to install IE 4.0 in order to operate. I declined. It looked so 1990s too. Wow.
Well that was fun.
- Dennis