But Vacuum Fluorescent displays look very fine and provide good brightness for any viewing angle. Ok, its difficult to build the required controller yourself, but it's easy to use industrial modules with embedded controller and serial (maybe I2C) interface.
Since the project idea interest me, I found some time tonight to think/dream about it
It might be a good idea to implement an electronic overcurrent protection using a counter and a timer. This is near to the solution that I found many years ago as I built an 8x8 matrix display with character generator using an Eprom and a counter. However lets get back to your project...
Did you notice that you can buy LED matrixes of 7x5 size? They are commonly used in elevators and in trains. If you connext a display with 7 rows to an 8 bit I/O chip, you have one bit left. What can we do with that one bit?
Think about a column counter with decimal output (such as CD4017, also called Johnson counter). You could use the free I/O line to clock that counter. Using two 7x5 LED matrixes, one PCF8574 and one CD4017, you can build a 7x10 matrix. You would use 7 of the 8 I/O lines to power the LED rows (through transistors, because of the high current), and use the free 8th line to clock the counter. Whenever the clock line pulses, the counter switches over to the next column. This principle is used there: http://www.best-microcontroller-project ... splay.html
You need a reset signal, to synchronize the hardware column counter with the NXT software since you would have a variable that must contain the current column number. When using a single PCF8574, there is no line free to connect to the reset input of the counter. But also remember there was a situation that we need to avoid: The counter must not stop for a longer time (>10ms) otherwise the LED's would burn.
Build a timer cirquit that detects when the timer clock fails (when the pulse is missing for more than 10ms). That timer can be used to reset the counter and it can also be used to switch of power to the whole LED matrix. When the NXT computer starts pulsing the clock line, the timer switches power on and resets the counter. Then the counter starts counting the columns starting with the first column.
I assume that you can use a 6x schmitt trigger chip such as CD40106 and some R/C combinations to build the required timer.
So you may use two 7x5 LED matrixes to get a 7x10 display, one PCF8574 (I2C I/O), one CD4017 (Johnson counter), one CD40106 (6x Schmitt Trigger) and a handfull of transistors, resistors and capaciators. If you need a larger display, then simply multiply the whole circuit, e.g. double it to build a 14x10 Matrix.
By the way, instead of using lots of transistors, you can also use driver chips of the ULN series. For example an ULN2804.
How does that sound?