I do the same thing someone else mentioned. When I'm trying to get metals I send all my toons that I can send on missions for that category. I do all the gift missions and fabrics missions to get the metal missions.
The downside is, even if I get a few Underworld Metals missions, chances are I'm still not going to get the one metal I need which in most cases is Mandalorian iron. I've sent my comp that adds +15 UT or however much it is to do a rich yield mission and still didn't get mandalorian iron. To make matters worse, these missions at this level take at the least 40-55 minutes to complete with high affection. I usually send Vette or my ship droid to do these missions and it's a crap shoot if I get anything.
Best thing to do that I've found is to buy the wealthy crew skill mission from the GTN and send your comp out for an hr and 45 minutes to do that mission. Buy another one and do the same thing again. If you have slicing you may be able to get them easier without paying as much as people charge. But then those can be even more random.
You forgot the part about loading a new instance once you sent your companions out to check for any UT metal mission, If you get one, cancel a mission your comp is currently running for it. Also, +15 efficiency means that companion will run it faster. You want a companion with +x critical.
As I see it, there are two problems with what you have stated.
The first is that the current system does not stop people from getting the available missions they want, it simply makes doing so frustrating and annoying. Making things frustrating and annoying simply turns players off and is not good game design.
Better design would be to allow players to directly select any possible available mission in a given tier, and balance the supply through the number of concurrent missions available and the mission returns.
The second goes back to balancing mat supply with mat demand. Even if we assume the probability of any given mission appearing in the list is the same as any other mission, we still are left with the fact that metal has a significantly higher demand than cloth, yet does not have a higher number of possible missions.
Solutions here include increasing the number of metals missions so the ratio of metals to cloth matches the demand / usefulness to simply removing cloth and having all crew skills use metals to chainging all force crafted items to use cloth and all non-force crafted items to use metal.
My simple solution can be done right now. Yours involves changing how the crafting system works. I don't disagree with you, we're just not answering the same question. Mine answers, what can I do about it right now. Yours answers, what can Bioware do to make it better.