Have you tried Show?
dp = DensityPlot[a r + b, {x, y} \[Element] Disk[], Frame -> False];
DynamicModule[{pt = {0.5, 0}},
Show[dp,
Graphics[{
{Thick, Circle[]},
Line[{{0, 0}, Dynamic[pt]}],
Locator[Dynamic[pt, (pt = constrain[#]) &], None],
{EdgeForm[Thin], Opacity[0.05],
Disk[{0, 0}, Dynamic[pt[[1]]]]},
{PointSize -> 0.018, Point[Dynamic[pt]]}},
PlotRange -> 1.1]]]
The Dynamic stuff does not work.
