Ah....
But Maemish ...Pythoneye isn't saying and never said can't...
You are.


All Pythoneye and I are saying is...
micro usb ports were never built for the singular load of ONE port handling continuous power, battery charging, and everything else thrown at it ...monitors and / or vnc and / or wifi or bluetooth and / or mice and / or keyboards ...and / or os's that then need to be so light as to be past DSL (damn small linux) ....
JUST to run on this idea you have put together.

And no one has insisted you must "believe them" ...here...
The EXACT same work has been done in this attempt...before..with other devices...(my last example was maru and buntu convergence on the Nexus 5 with slimport cabling..)
This isn't about believing anyone...
I could give a tinkers damn if you "believe me" or pythoneye..
The science ...the work that has been done...the trial and error done..all has been measured inside the parameters of the tech existing...and the verdict ..for now ...is in.
Believe all you wish ....believe the earth is round...believe it is flat...
Your belief doesn't negate the findings.
Teams of devs have worked on such ideas with the micro usb port and utilizing slimport -esque cables ...
all kinds of optimizing ...
done exactly what you wish...
But the cost to attempt the same for the n900 may be too high.
and even then...
as I said...the best with all that running ..and microusb charging ...
will be a battery drain...I don't think you will maintain a holding pattern ...
No one else can ...

And this isn't a "can't" issue...
This is called no option but to work inside the box...
within the parameters of the known limits of the physical equipment that cannot be exceeded

And believe me ...
you aren't the first to have thought of this idea in our forum...
I have as well...
The only difference is ...
I would ...except for ports and their necessities ...in the body ..
and the battery...gut all else...and put another battery in the cavity..
and somewhere shove in the smallest stick pi there is ..as well..
and the only difference between your new idea
and my old idea is...
my idea may work.
Mainly because you are stuck on the n900...

Whereas I ...I am stuck on the n810

"The n810 Tank" ...solid build....separate power ...separate usb ...
may still have a drain on a couple of battery...
But I bet you can run much more...much longer.
You may even get a holding pattern or even actually charging ....

But you will most likely have to drop the idea of micro usb as the "carry all" ...and have separate power port ...instead ...to help.
Micro usb was never really designed for such load ..

But...its your choice what and how you do what you do...
and I wish you luck with the endeavour on the n900.