R – how to obtain the client id of a ContentPlaceHolder in an easy manner

I think you are attacking it from the wrong angle by trying to encode all posted data.

Note that a "<" could also come from other outside sources, like a database field, a configuration, a file, a feed and so on.

Furthermore, "<" is not inherently dangerous. It's only dangerous in a specific context: when writing strings that haven't been encoded to HTML output (because of XSS).

In other contexts different sub-strings are dangerous, for example, if you write an user-provided URL into a link, the sub-string "javascript:" may be dangerous. The single quote character on the other hand is dangerous when interpolating strings in SQL queries, but perfectly safe if it is a part of a name submitted from a form or read from a database field.

The bottom line is: you can't filter random input for dangerous characters, because any character may be dangerous under the right circumstances. You should encode at the point where some specific characters may become dangerous because they cross into a different sub-language where they have special meaning. When you write a string to HTML, you should encode characters that have special meaning in HTML, using Server.HtmlEncode. If you pass a string to a dynamic SQL statement, you should encode different characters (or better, let the framework do it for you by using prepared statements or the like)..

When you are sure you HTML-encode everywhere you pass strings to HTML, then set ValidateRequest="false" in the <%@ Page ... %> directive in your .aspx file(s).

In .NET 4 you may need to do a little more. Sometimes it's necessary to also add <httpRuntime requestValidationMode="2.0" /> to web.config (reference).

There is nothing like SharePoint ContentPlaceHolders & non-SharePoint ContentPlaceHolders. When you add a Default content to the Master Page ContentPlaceholder and if you dont override it in the Content Page then it will be visible in all the Pages that use that Master Page. Only way to get rid of it is to Override those Content Place Holder with empty content in the Content Pages. As overriding contentPlaceHolder in all the Pages is not an good Aproach. What I recommend is not to place conent in the ContentPlaceHolder of the Master Page but instead , put the content in the Content Control page of the Pages, so that Content doesn't come in all page but only the page that needs it. Yes it not a good to put the exact same content in multiple page. but in your case that is only the Option. May be you try to wrap the content in to a User Control so that the code can be updated only one place