I think RD is considered pretty fast burning and/or high pressure for 20 gauge. If you can't find published recipes on Alliant's site, I would stay away from it.
There were some guys on Shotgunworld who created some 3/4 oz and 11/16 oz loads for the 20 Gauge using Promo. They had them pressure tested too.
Here they are:
20 GAUGE LITE Hull: AA-HS or any Remington Shot: 3/4 or 11/16 oz. (Stock MEC charge bars. Actual average dropped: 3/4 oz. - 315 gr.; 11/16 oz. - 300 gr.) Powder: 12.5 gr. Alliant Promo (or 15 gr. Unique or Universal. You'd have to determine which MEC bushing drops that.) MEC Powder Bushing for Promo: No. 21 Primer: Fiocchi 616 (209), W209 or STS209, Rio, Nobel Sport Wad: Current "new" WAA20 (The older WAA20 wad and others won't crimp well.) ~ Average Velocity: 1232 High Velocity: 1242 Low Velocity: 1227 Extreme Spread: 15 Standard Deviation: 4.9 Pressure: About 9,500 psi using Promo (Tested). Less than 11,000 psi using Unique. NOTE: This unpublished load using Promo was cooked up by some of us here on SGW. Burns cleaner than any load I've ever used in any shotgun, no matter what gauge/bore. May or may not cycle semiautos.
Billy, as noted previously, Red Dot is a bit hot for the 20 gauge, although Hercules' reloading guides listed a few 20 gauge Red Dot loads for many years in 3/4 and 7/8 ounce payloads. Looks like Alliant dropped them all from their guide by the 2004 edition.
I'd be nervous about using Red Dot data to load Promo in 20 gauge applications without some pressure tests. I've read the statement that it is the same burn rate as Red Dot, but also note that it is denser. Don't know if that's important to the internal ballistics of the load, but that seems to be a variable to consider. There may be a good reasons that the Alliant folks only list Promo as a 12 and 16 gauge powder.
Red Dot 20 gauge pressures crowd the upper end of SAAMI specifications. There isn't much margin for error at the top of the pressure curve.
That said, I do have a favorite Red Dot 20 gauge load from the recent Hercules guides. One advantage of fast burning powders is a slight, but noticeable reduction in muzzle blast. Especially in short-barrelled quail guns. 7/8 oz lead 2 3/4" Remington RXP hull WWAA20 wad CCI 109/209 primer 12.0 Red Dot 1155 fps/11,300 psi
There are more recipes in any of the 80's or 90's Hercules reloading guides if you can scrounge one.
Alliant did have 3/4 oz data in years past. RD, GD, and Unique were used for this data. I emailed them awhile back asking why it had been discontinued from the manual. Their answer was: lack of interest. They said it was perfectly fine to continue using that data.
I use Promo (RD) for some 3/4 oz 20s, though I prefer GD. A friend had the load pressure tested by Tom Armbrust and it was quite acceptable.
I use the same load data (plus some puffed rice) for 5/8 oz 20s that my 10 y/o son runs through the 391. No, they don't cycle the action.