We created a library that abstracts away a lot of the 'hard stuff' for creating a Custom Fields-like plugin into a library called BugField (http://www.fogcreek.com/FogBugz/library/70/html/135BB8DE.htm). If you are interested in doing some kind of custom UI, that would be a great place to start. You'd want inherit from CLongTextBugField and override BugView.
Here is the code for a plugin (happens to be written in Wasabi, but if you're a coder you'll get the idea) that uses BugField. As you can see, most of it is just establishing the identity of the plugin.
If you're looking for a solution that doesn't require any coding, I have added a feature request to get this into Custom Fields, and we'll try to get this into a default later version.
Public Class BugFieldTestPlugin _
Implements IPluginBugDisplay, IPluginBugCommit, IPluginFullTextIndexBug, IPluginGridColumn, IPluginFilterOptions, IPluginFilterCommit, IPluginFilterBugEntry, IPluginDatabase, IPluginSearchAxis _
Inherits CBugFieldPlugin
Protected Static PluginId = "BugFieldTest@fogcreek.com"
Public Sub Class_Initialize(api), Base(api) : End Sub
Public Override Function DatabaseSchemaVersion
Return 1
End Function
Public Override Function BugFields
Return Array( New CLongTextBugField(api ,"BugField Test LongText", "Long Text Test", PluginId, "longtext_col", 6))
End Function
End Class