Greensand

"Used as a soil amendment and fertilizer, Greensand (0-0-3) is mined from deposits of minerals that were originally part of the ocean floor. Contains about 3% total potash, along with iron, magnesium, silica and as many as 30 other trace minerals. May also be used to loosen heavy, clay soils. Improves plant health. Ideal for use in organic gardening"

It's a very very slow release form of potassium but is used more as a trace mineral supplement.

^^^^^^That

Greensand compliments kelp meal quite well and vice versa.

Wet

It's good to use if you re-use some of your soil as well, as it's slow to break down. It can only be used in your mix, it's not for teas or anything like that. It will also weigh down your mix as well. I use a little in my mix mostly just to keep the soil a little heavier when the pots are almost dry. Don't want my fan blowing those bitches over.

Organic form of SILICA " bud steroids" will increase bud density in moderation but in LARGE quantities or in synthetic form added late in flower can leave you with buds that won't burn all the way and leave "exoskeletons" of nuggets when smoked. With the sands, its such a slow release and organic product that it is almost always beneficial, beware however than some of the cheaper green sand products can leach your Iron and Mag out of your soil. Can cause Chlorosis which is a lack of chlorophyll in the leaves "yellowing of lower leaves spreading up". Make sure to use a calcium/iron/mag supplement when using the sand.

Would adding a 5 lb bag of epsoma green sand to a full mix of supersoil (@65 gallons) be of any benefit? Or would the proper way to use greensand be to top dress late in flower?

Yes, it would be of benefit, but you don't have to use the whole bag for 65 gallons. You could split between two batches of super soil. Don't topdress late in flower though. That wouldn't be beneficial at all.