Attenuation and mouth feel do not have a 1:1 relationship.
See the French Saison strains for an extreme example; there are few saccharomyces strains that have higher attenuation and will dry out a beer more but beers made with this yeast strain still have a nice creamy mouthfeel from the yeast's production of glycerol.
Attenuation / Final Gravity is only one factor.
A great article from the Journal of the UK-based Institute of Brewing and Distilling that goes into the physical and chemical parameters which may contribute to beer mouth feel:
http://onlinelibrary.wiley.com/doi/10.1002/j.2050-0416.1993.tb01143.x/pdf
It's interesting that that article mentions that high molecular weight proteins and specifically the colloids formed from them are highly important for beer "mellowness" and "palate fullness". -The most famous such colloid would be chill haze; something that may certainly be present in Hill Farmstead beers given their cloudiness and huge hopping levels.
-US 2 row malts have much higher protein levels -as protein conversion continues in the mashtun we see large proteins break down into peptides and eventually amino acids. Doing a high temp single infusion mash with typical high DP US 2 row -and doing a shorter than normal mash would both keep more less fermentable staches around (Dextrins) AND more of these larger proteins around -we know that thinks like Flaked barley, which has been mentioned on this thread quite a few times now will also introduce more protein. Having a lot of these large molecular weight proteins AND dextrins AND then chill haze colloids that form by having those large molecular weight proteins in solution in the presence of large amounts of hop-derived polyphenols WOULD result in lots of protein/tannins colloids being formed.
-De Clerck and Narziss both agree that higher protein malt will increase the amount of high molecular weight proteins in the finished beer so I find it super interesting that combining crazy american high hop levels along with our high protein / enzyme malt (which europe considers inferior for brewing and I'd generally agree for all malt beers, especially strong ones), would be uniquely well-suited to creating these very silky / pillowy yet extremely hoppy beers. -If anything should be an American beer specialty, this is it.
On the subject of Beta Glucans / Oats, the article seems to agree with another poster that felt that flaked barley helped out much more. Beta Glucans certainly add to the viscosity but seemed to have only a mild coorelation to "sensory viscosity"; personally I definitely find an improvement, but a trained sensory panel is hard to agree with; I get the feeling that they're making pretty detailed distinctions.
Adam