OK to put comments before the XML declaration?

No, it's not OK.

Appendix F of the XML spec says:

Because each XML entity not accompanied by external encoding information and not in UTF-8 or UTF-16 encoding must begin with an XML encoding declaration, in which the first characters must be '< ?xml', any conforming processor can detect, after two to four octets of input, which of the following cases apply.

Ah, but, section F is non-normative, you say.

Well, section 2.1 gives the production for a well-formed XML document, thus:

[1]     document       ::=       prolog element Misc*

...and in section 2.8 we get the production for "prolog":

[22]    prolog     ::=       XMLDecl? Misc* (doctypedecl Misc*)?[23]    XMLDecl    ::=      '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'

So, you can omit the < ?xml declaration, but you can't prefix it with anything.

(Incidentally, "Misc" is the category that comments fall into).

according to this page, this is illegal:

When adding reference comments to your XML code, remember that they cannot come at the very top of your document. In XML, only the XML declaration can come first:

<?xml version="1.0"?>

The XML declaration specifies the document encoding, which is as important for comments as for structural XML. Therefore, the declaration should go first. I wouldn't be surprised if many XML readers were able to deal with this, but it's a bad idea.