yes, I can see that this would be a solid way to ease hoh farming when b2/b3 can take up a good portion of a valuable weekend. Although, I'm not entirely sure its the most efficient when considering energy costs. Efficiency of course isn't the end all, be-all way to play though if a player values other factors over it, such as time, etc.
For me in 10vs10, 200 guild points is roughly 2-3 battles of 3x successful attacks each along with the guild winning the attack. Each attack takes ten energy. I usually average 90 points per battle, so it would take 70 energy to get 10 almighty pieces (2 battles x 6 attacks and 1 more attack from 3rd battle, will also give some leftover guild points). I admit this is fuzzy math and based on personal experience and blatant laziness (lol) to get the exact points researched, so take it with a grain of salt, along with the next paragraph based on it.
Hall of heroes, I can auto b2 with no issues, and maybe b3 but choose b2 because its a lot less time per run and also a 100% success rate. It costs 6 energy per run. Data shows each run of b2 gives on average 1.5 pieces. Assuming the average, It'll take me 200 energy for 50 pieces and 160 energy for 40 pieces, so 40 more energy for the last ten pieces farming hoh vs. 70 energy worth of guild points, so a net loss of 30 energy in terms of officiency. In a worst-case scenario of getting just 1 piece a run, it will take 300 energy for 50 pieces, or 240 energy for 40 pieces. In this situation, its less efficient by ten energy if we make up the difference with almighty pieces. Certainly not a big deal imo.
all of the above doesn't detract from the possibility that this may still be a very good idea. We can save a bit on crystals by using less refills in proportion. After people buy the pieces needed to summon their random tri-element ifrit (s), aside from 4* angelmons, they have a huge range of options. Throw in the fact that guild wars can be done all week, hoh comes once a month and you only have three days to farm it, and after getting ifrit out of the way, you could buy rainbowmon and either almighty pieces or something else with your points.