This example shows how you can display faders on a dialog. It also shows how a resize corner can be included to allow the user to change the size of the dialog. I've attached the LUA script for this.
There appears to be a lot of UI controls available for creating dialogs.
For example, take a look inside any of the "*.uixml" files that can be found below "C:\ProgramData\MALightingTechnology\gma3_1.5.2\shared\resource" on a Windows onPC installation. These files appear to contain an XML definition of the dialogs used by the MA system. This is a useful source of information regarding the names of the available UI controls and their properties.
Also, if you look at some of the .lua files below that path you will find examples of the signalTable being used to respond to UI events.
Hopefully that will be of help to others.