In a situation where raw HTML is allowed, there are special tags that let you specify certain special characters. There's a generic tag that will work for any character for which you know the ID number, and there are special-purpose tags for the common international characters such as what you're looking for. It's better to use the special-purpose versions when you have the option because they render correctly independent of the character set being used by the reader's browser, and because they are (I think) easier to remember. Examples:
To figure out character numbers, use charmap.exe in Windows as previously mentioned, or the refer to online reference card at Visibone. No doubt Google/Wikipedia would offer additional resources. If you study the Visibone charts briefly, the mnemonic tags should make sense. The macron characters aren't actually included on the charts, but the macron mark itself is. I tried using the ¯ tag immediately before and after different letters, but wasn't surprised to find that placed the characters next to one another rather than overlaying them. So I decided to see if just following the pattern established in the tags for the other similar marks would work, and sure enough it did.
Whether or not you can use these tags on sites like LJ an FB depends on how "helpful" their text entry forms are. Many characters that we frequently type—such as quotation marks, ampersands, and less-than/greater-than signs—have technical meaning in HTML. Often what's "code" and what's "just text" is clear enough from context, but otherwise we must/should use special tags like these to indicate when special characters should be interpreted as "just text" and not part of the markup syntax. So that we "dumb users" don't have to worry about such things, any special characters we enter into text boxes are usually "encoded" automatically when we submit the entry/comment. This requires some guesswork behind the scenes, and overly aggressive encoding algorithms may assume unusual tags were meant to be "just text," and prevent them from being interpreted properly. LJ comments disallow most HTML unless you use the "Don't auto-format" option, which isn't visible until you click the "more options" button. LJ journal entries allow some HTML even when using the pretty formatted text version of the editor, and most any tag in the HTML version. The <lj-raw> tag or the "Don't auto-format" checkbox can be used in the HTML version to make it accept any standard tags.