High-current power/data one wire bus?

Not sure if the small 16F parts support it. Would also give you enough pins
to drive multiple colours of LEDs at a node.

No

OK, that is a similar system to what I am suggesting except you are clamping
the voltage right down close to 0, which means you can use voltage sensing
at the power supply end. However this will mean that you will need to have a
power supply that has a sharp knee on its current limit, possibly even some
form of foldback current limit, and the pass element has the possibility of
getting very hot - remember this will have the whole power supply input
voltage across it while the node is shorting to ground, ant will need to be
current limited at a touch over 2A for 100 nodes at 20mA/node - want expandability. Not only that but the transistor at the node will have
to have this current capability as well. Note that unless you have some
large capacitance at each node, isolated from the supply by a diode, you may
see the transmission modulate the light level of any LEDs that are turned on
due to the chopping of the supply voltage.

The current modulation scheme is similar at the node end, except it uses a
higher value resistor, so the node can turn on sufficient current to be able
to be seen at the power supply as a current fluctuation. Not only that but
remember you have other current fluctuations caused by LEDs turning off and
on.

...

But you will want a checksum if you start doing firmware downloads.
Depending what you are using the LED display for, you may still want a
checksum of some sort, there is nothing more frustrating than having such a
system go AWOL in sequence, even if it is only a demonstration Christmas
tree lighting display.

...

I would see if I could find some dark green sheath CAT wire - like used on
Christmas tree lights. As to the component cost, I would try hitting up some
FAEs for chips - I am sure you could get a couple hundred CAN drivers & PIC
chips for 'demonstration purposes' especially if you are prepared to write
an app note of the experience (student task - part of the project!). You
might even get the same consideration for a roll of CAT cable with a special
coloured sheath - would the army use CAT5 with camouflage green sheath? With
the right colour I think you would find that even CAT 5 would get reasonably
invisible.

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