If that's all you want to do with the "Register" link, and if you are using only one theme, and not allowing users to change themes, you can simply edit lang/english.php (or the language file your board uses). You will find the text for the Register link on line 127 in english.php.
'register_lnk' => 'Register',
you can change it to
'register_lnk' => '<b>REGISTER</b>',
You could also put span tags around it, if you'd rather do it that way. Of course, this will work only for the language you primarily use for your board. If your users are multilingual, and you allow the user to choose other languages, editing all would be pain. If that's the case, you would need to make some changes in theme.php for the theme(s) you use.
And no, you can't just save it as .html - the script won't work!
Adding text to the input registration page is going to get you into editing the code, since that page isn't customizable through themes. What you could do as an alternative is add your notice to the top of the registration agreement, perhaps in bold letters, or red text or blinking, for that matter. If you want to try that, go to lang/english.php, and look for this code (line 1476):
$lang_register_disclamer = <<<EOT
While the administrators of {SITE_NAME} will attempt to remove or edit any generally objectionable material as quickly as possible, it is impossible to review every post. Therefore you acknowledge that all posts made to this site express the views and opinions of the author and not the administrators or webmaster (except for posts by these people) and hence will not be held liable.<br />
<br />
You could change that to something like this:
$lang_register_disclamer = <<<EOT
<b>**NOTICE** (put what you want to say here)<b /><br />
<br />
While the administrators of {SITE_NAME} will attempt to remove or edit any generally objectionable material as quickly as possible, it is impossible to review every post. Therefore you acknowledge that all posts made to this site express the views and opinions of the author and not the administrators or webmaster (except for posts by these people) and hence will not be held liable.<br />
<br />