Here's a list of the currently editable fields. If ESGME doesn't understand a field (Tag) it will leave it untouched and unchanged when you save your file. So, don't worry if you have custom tags, they'll be respected.
<game>
...
<desc>...</desc>
<developer>...</developer>
<favorite>...</favorite>
<genre>...</genre>
<hidden>...</hidden>
<image>...</image>
<kidgame>...</kidgame>
<lastplayed>...</lastplayed>
<marquee>...</marquee>
<name>...</name>
<path>...</path>
<playcount>...</playcount>
<players>...</players>
<publisher>...</publisher>
<rating>.</rating>
<releasedate>...</releasedate>
<video>...</video>
...
</game>