Would you answer these questions, so I look into the issue? I'm mostly a handcoder, so what you are doing is possible, I believe.
0)Explain your setup config. I'm not sure what you mean by you are not running 'localhost'. Are you running DNN on a local machine or remote hosted environment?
1)What version are you using?
2)What simple change are you making?
3)What skin are you using?
I run DNN on a workstation, with the naming convention:
virtual dir name: dnn030101_16
physical path: c:\dnn030101_16
database: sql/msde
framework: asp.net v1.1
browser: iev6
I used a text editor to add a simple table row with a text welcome message.
Adding the following code to the \admin\security\signin.ascx file (in bold):
<table cellspacing="0" cellpadding="3" border="0" summary="SignIn Design Table">
<tr>
<td colspan="2" class="Head" align="center">Welcome to InfoCentre</td>
</tr>
<tr>
<td colspan="2" class="SubHead" align="center"><dnn:label id="plUsername" controlname="txtUsername" runat="server" text="UserName:"></dnn:label></td>
</tr>
I open a new browser window and wait for the JIT to compile the controls, and the change shows up.
I think we can help sort this out easily enough, by understanding your setup and what you are doing.
Cheers...