I don't know anything about the configuration GB posted about, but ever since the Encore first came out some custom 3rd party shops have made and sold a rimfire breech plug adapter for the Encore with an angled firing pin to shoot the rimfire cartridges. With it installed in place of the factory breech plug as many different rimfire barrels can be shot on that frame as wanted (22S/L/LR. 17HM2/17HMR, 5mm, etc). But you do have to change out the plug back to the factory one to shoot centerfire on that frame again.
I don't pay ridiculous retail prices at most sporting shops. Mail order 17HMR ammo can still be bought for about $9.50-11.00 per 50 from the cheaper to the so called premium brands, and the 17HM2 for about $5.50-$6.50 for the same. Deals come up now and then too at even lower prices - couple of years ago I paid $4.50/50 for a couple of cases of HMR from a small shop in WA state, and later $7.00/50 from a shop in northern NV (took all they had). Since I also bought several cases of both 17HMR and 17HM2 mail order back when they were still less than $8 and $5 respectively, they'll last me for years now that I don't shoot them like I used to.