This is going to seem primitive to you sophisticates, but here goes. Any Storm Petrel sailors out there? You too can have water ballast! I don't need to tell you that SPs are skittish and tend to heel a bit too readily, especially single-handed and in a breeze (Alison's remarks about reefing in a dinghy are highly relevant to SP sailors). I now use camping water carriers, the type which are flat when empty (can be rolled up into a small space) and carry 10 litres each, which is a manageable weight. Two, four or six of these (= 40 - 120 metric pounds of ballast) depending on conditions (usually four), stowed either side of the dagger board case, are low in the boat, aren't in the way and have a wonderful effect on stability and handling.
Not nearly as flexible as in Swallowboats with proper tanks, but very useful. It may be obvious to others, but I learned the hard way that you can't just dunk a flexible water container under the surface and expect it to fill, because it won't (if you don't see what I mean, try it!). The solution is a cheap gardening watering can, with about a foot of garden hose attached to the spout, in the boot of the car. I fill up the water ballast containers, rig and launch the boat, load the water, and off I go. Reverse the procedures at recovery after the sail.
Like I said, a bit primitive, but workable and very helpful.
Michael Rogers