I do not know about making whole new windows, but editing existing windows is perfectly doable. There is a wealth of information on how to do it at EQInterface.com.
You can then edit in some stuff to use MQ information. Basically you will tell the UI to use a bogus EQ information piece (thus when you do not have MQ loaded your UI will show a bunch of "unknown" tags where the MQ stuff is suppose to be) but would otherwise treat it like other EQ pieces as far as positioning, size, additional graphics, ect ect.
I have a few pieces you are welcome to look at
here. Namely it is just enhancing the target window, group window, and XTarget window. (the other pieces do not have any MQ enhancements, they are just stuff I like) I am by no means a XML expert, I just took time out to look over stuff and tinker with the code enough to get what I wanted to work well enough for me.
an example would be like
<Label item="MQ_TargetClass">
<ScreenID>MQ_TargetClass</ScreenID>
<Style_Tooltip>false</Style_Tooltip>
<EQType>9999</EQType>
<Font>2</Font>
<TooltipReference>${Target.Class.ShortName}</TooltipReference>
<RelativePosition>true</RelativePosition>
<Location>
<X>29</X>
<Y>23</Y>
</Location>
<Size>
<CX>99</CX>
<CY>15</CY>
</Size>
<Text>Warrior</Text>
<TextColor>
<R>255</R>
<G>255</G>
<B>255</B>
</TextColor>
<NoWrap>true</NoWrap>
<AlignCenter>false</AlignCenter>
<AlignRight>false</AlignRight>
<AlignLeft>true</AlignLeft>
</Label>
and of course you need to add it in the list of pieces at the bottom IE
<Pieces>T_Label</Pieces>
<Pieces>MQ_TargetLevel</Pieces>
<Pieces>A_TLVL_Label</Pieces>
<Pieces>MQ_TargetClass</Pieces>
<Pieces>MQ_TargetX</Pieces>
<Pieces>MQ_TargetY</Pieces>
<Pieces>MQ_TargetZ</Pieces>
<Pieces>MQ_TargetDirection</Pieces>
<Pieces>MQ_TargetDistance</Pieces>
Basically how I learned to tinker with UI pieces was researching EQInterface first, and tinker around with some basic moving stuff around... changing sizes... colors.. ... after that I added additional info feeds.... once I felt I had those down I took a look at the existing MQ enhancement UI pieces from the
source site to see how other folks did it. Then I tinkered around with getting stuff set up how I wanted it to be.
I will give far warning up front, the XML code is very unforgiving on syntax errors. If you forget a / or mistype a label, the entire UI crashes to default. And it takes a while to load... crash... then load default... before you can see whats what. IE it can be very frustrating to work with. I have not been able to find a working "what you see is what you get" UI editor, and the existing XML editors can be a learning experience in and of themselves. I use Notepad++ with XML highlights, but what ever works for you.
Something that is not obvious (except in retrospect) but may be helpful is a file inside your main EQ folder ( the same one with eqgame.exe) called UIErrors.txt, which will show what errors EQ tosses up while loading your UI. Now when you load a custom UI, if you do not have for instance a custom inventory piece in your custom UI folder, if will toss up an error and load the default IE
Warning: file EQUI_BankWnd.xml not found in directory UIFiles\stuff\. Attempting to use file from Default skin.
this is normal. Any UI piece you dont stick in your custom UI file, it will grab from the default folder. How ever if a certain UI piece is tossing up an an error, and you are not sure which one ( say for instance you are trying out one of the full blown UI sets) this can help you narrow down while piece is giving you issues.