Sure, but you're either going to have to generate the whole font up-front(can be many thousand characters) or you need to re-upload as you use/generate new characters which can thrash unless you're very careful about the regions you lock(and you have a driver that behaves appropriately).

Most font renderers I know do a tiered LRU cache of 3-4 texture "pages" which hurts your drawcall batching but tends to be a nice tradeoff in texture usage.