Re: Egg Tempera and D.S. Watercolour Ground
You could also try out practicing with egg tempera directly on your 300 lb watercolor paper. When I first tried egg tempera I made some sketches in my Moleskine watercolor sketchbook (fairly thin paper) and they came out fine. They're still fine more than a year later, though they don't have very many layers of paint on them. I read that egg tempera tends to become brittle over the years, so finished works on paper need to be kept from bending.
(I haven't tried the Daniel Smith watercolor ground, so I can't comment on that. I don't think watercolor paper needs any extra sizing to receive egg tempera though.)
I know that the experts recommend only painting egg tempera on traditional gesso/rabbitskin glue on a rigid support, but for just practicing, plain paper is fine, and you'll start to get a feel for the way the egg paint handles before investing in fancier supports.
I have done a couple finished egg tempera works on Ampersand Claybord. There are mixed opinions about the longevity of egg tempera on Claybord. However, I feel I'm still kind of practicing, so I'm not too concerned about using Claybord. It's widely available and pretty cheap, and from what I've heard, Ampersand has recently changed its formula, and concerns about the old Claybord no longer hold.
I've heard good things about the True Gesso panels Margarete mentioned (in the Watercolor forum), but I haven't tried them yet. (I haven't tried real gesso at all.) Some people say the initial layers of paint go on a bit differently on Claybord as opposed to traditional gessoed panels, and some people who are used to real gesso panels don't like Claybord. I'll say that the first few layers do seem really weird on Claybord (they get sucked in and look a bit blotchy), but it's fine after those first few layers.
I'm sure the more experienced egg tempera painters will post responses pretty soon. This forum just doesn't get visited very frequently.