Yeah man, I'd love to see custom goalie masks soon. I could be wrong, but I think the reason why it's not included yet is because goalie masks use a texture map versus a color id map that they seem to use for the other gear to identify each zone. Or maybe the UVs are separated and dedicated to the colors we choose.
Anyways, all of that to say that EA could probably setup 5 different goalie masks where you could choose colors for dedicated zones and probably put your team logo on the sides or chin of the mask. The next best option, which would take a lot of work, would be setting up something like the creation area in Gran Turismo where you can apply shapes and gradients on layers of the model and move them around or rotate them. I think it's unlikely that we'll be able to upload our own texture maps for goalie masks because someone would need to monitor the content. I do think whenever they add custom goalie masks, it'll be something refined instead of basic.