Call your local LQSs; many carry Synthrapol now.
Synthrapol has to be used in a *lot* of hot water. Don't try to use it in a domestic front loader; there isn't enough water. A laundromat front loader is okay, especially the big ones. A domestic top loader has enough water. The need for water is because, if there are loose dye particles, there has to be enough water for them to stay dispersed in; if water quantity is inadequate, extra dye particles will still settle in fabric. (As long as they are unset, they can still be removed in subsequent washings in Synthrapol, but who needs the agro?)
If you are using a top loader and want to be very "easy" on the quilt, do not allow it to agitate; instead, stop the machine during the agitation cycles and hand agitate by pushing down on the quilt, then skip to spin cycle. Since the water has to be hot, you probably want to wear thick rubber gloves for this to protect your hands.
I have read the Dawn substitute thing online for years but never could find the clear Dawn to try it out (*must* be the clear Dawn that looks like water; cannot be the blue Dawn!). Just recently I finally saw clear Dawn at Walmart.
Chances are your fabrics won't bleed, but using Synthrapol provides insurance.
Edit: I just did a quick Google search of websites for using Dawn as a substitute for Synthrapol. They all seem to be for dyers and specialized applications, such as washing silks prior to dying, or using a drop as a surfactant for dyes. There's no guarantee that Dawn will work the same way as Synthrapol does in terms of suspending unset dyes in water when washing a quilt.
Also, just wanted to mention that color catchers are fine for slight dye leaks but will not be enough if there is a major dye bleed when washing a quilt. It's fine to add color catchers to a Synthropol wash, but they will not provide the same amount of "insurance" that Synthrapol provides.