No I do not support the letters yet, but i'm looking into it soon (planned middle/end of february) With the reason I got another mod to update and I've examens right now...
Maybe I'll support static letters, or takes the biggest letter signal that is on that tube, still thinking of a way to encorporate multi-letter displays, as in, only the right one needs to be hooked up and let a full word be displayed... (Feel free to submit ideas)
although the above question was asked 7 months ago, here is some comment to it since probably i soon will build qute a few displays which should be quite responsive to show lots of current status data ...
about negative numbers: missing negative numbers are not a big problem if it is easy to see that a number is negative. that could be achieved by adding an additional colored lamp to each number (ugly and quite a lot of additional circuitry), or by simply having a second color for negative numbers (no full color support, but only one second color, eg red tubes for negative numbers).
about general color support: when nixies were new i used lots of colors for fun, and later it was nice to have displays colored according to item types (different colors for each item were done by another mod), but since that once again required lots of additional circuitry for mixed signals (because of color priorities, etc) and because not all items had colors, i stopped using such "colored by item" displays and now use at most green/red to indicate good/bad (eg positive/negative) numbers.
about word displays: some other mods (probably for train station names etc) use encoded text, eg A=1 for A in the first position (I don't know whether this is the first from the left or from the right), B=1 for B in the first position, =2 for the second position, =4 for the third, =2^(N-1) for the Nth position. thus an entire word up to 31 or 32 characters can be transmitted on one wire, eg "A=9 (1+8), B=6 (2+4)" would write ABBA, or "a=2304 (256+2048), e=8, l=17 (1+16), n=512, o=2, s=128, t=1024, v=4, y=32" would spell your name :-)
it would even be easy to scroll the display by simply multiplying or dividing by 2.
and (although not yet tested) there is a mod DISK (https://mods.factorio.com/mod/disk) which stores 512 such snapshots of the state of wires at specific addresses in its virtual memory, ideal for storing up to 512 notes of a song (to be played by the speaker) or for storing lots of encoded text (up to 512x32 characters = 16 KB of text !!!)
here is the blueprint (only using vanilla plus alpha-nixies) to encode and decode strings. it would be nice if these two functionalities could be included to the mod so that we only need two entities instead of the whole setup with 94 combinators (for 32-char strings): an alpha tube, and a new constant combinator that doesn't output its signals directly but multiplies each of the signals with a weight (1,2,4,8,16,etc) depending on the location of the signal in the combinator grid (i don't know how to implement this, but several mods are doing similar, looking at the position of signals in the grid)
in this blueprint, a string can be setup by selecting one chaacter in each of the bottom constant combinators. they will be weighted and added, the encoded string then is visible at the wooden power pole at the left side, and that signal is decoded at the top and displayed on the alpha nixies.
there are three combinators located on stone floor. connect only one of them at the same time: the current connection is to the bottom combinator to get the encoder connected to the power pole. disconnect them and connect one of the two left constant combinators instead to see the two example strings (the values from my last message) decoded at the nixies.
if the lowest order bit is on the other end of the array (in the other mods which use this type of encoding; please check to be compatible with those strings), just chain and shift the combinators from right to left instead of from left to right ...
First of all, i'm tempted to rewrite the mod again from scratch, optimizing it further again.
The other issue that i'm having is the amount of sprites that i can fit on a single entity. For the letters i would need 27 sprites (26 + empty). This would mean i have to add 3 extra entities to each nixie and that will cause again more issues.
On the color part, combinators do not support colors, so I cannot set colors as i want. AKA for each color I need a new set of nixies, or change the entity to a less suitable entity, demanding more ups. I am looking at a workaround for this already.
This is still on my todo list, I am working the problems. It just takes time to think and find a suitable solution.