My 20 cents worth. Brazing/soldering titanium is an aerospace research topic. It can be done with silver/copper/aluminium filler like incusil ABA,VH720 and others, or very precious and expensive metals but it seems no ones sure about long term corrosion resistance of the filler material. The lugs would need to be titanium and it would have to be done under inert gas. I have thought about building one, getting some ti lugs machined but its not going to be easy, fast, or soon.
Steel tubes would be as light, particularly if you go for new, fancy heat treated pipes like columbus niobium SL, KVA MS2 or true temper platinum/VHT. Its moved on a bit since 531 special lightweight etc. Oversizing means more metal. As other folks have pointeed out, bonding to lugs intended for brazing/soldering doesn't have enough surface area to work because resins are nothing like as strong and therefore need more surface, this makes the lug longer and changes the bikes feel a lot, people who owned raleigh techniums out of ti and other materials said they sort of all rode the same. At least with bonding the lugs can be aluminium which is much easier to machine.