I did a little research. There is a 'wax' for printing for the investment cast process. It can be used on most any 3D printing machine with some basic modifications regarding 'feed', head temps and a few other things. Some machines are better than others for making waxes. I am not computer savvy and don't own any CAD software and machines or 3D printers. I am not a framebuilder (yet) but do make my own scale RC models. Some of them are sailplanes and are large with spans of 4.5 meter. I hand draw my plans after doing research, mold fiberglass, cast silicone rubber parts from my own patterns and molds, cut wood parts and machine metal parts some which require soft or hard silver soldering.
I don't know anything about costs and the question isn't about that, sorry! But... there are manufacturers who DO provide investment cast lugs so infrastructure exists for the process.
One business model: the customer could submit drawings for lugs then the fabricator could furnish the waxes, finished on the inside for tube fits, to the customer for final exterior finish sculpting. The printed wax items aren't the most smooth due to the thickness of the 'string' layers. When done the customer sends them back to the fabricator to be ceramic coated and then cast. I would think that framebuilders are artisans enough to deal with finishing wax parts.
Second model which would be on the opposite end of the 'custom' spectrum above: customer logs onto fabricator's site and chooses from a myriad of lug designs,windows and features and the fabricator would just print and finish the waxes for casting.
I'm sure there could be a whole host of scenarios but wonder if the framebuilder community is large enough to support it. I can only dream!