Question: Implicit 3d plot

Hi.  I want to plot the function U in terms of x and y,

U :=2*diff(p(x,t),x)*diff(q(y,t),y)/(p(x,t) + q(y,t) + a[0])^2;

where

p := -k[1]*(3+2*k[2]*k[4]+k[2]*k[4]*sech(k[2]*xi-k[3]*t)^2)*tanh(k[2]*xi-k[3]*t)/(3*k[2]);
q := -l[1]*(3+2*l[2]*l[3]+l[2]*l[3]*sech(l[2]*eta)^2)*tanh(l[2]*eta)/(3*l[2]);
xeqn := x = xi+k[4]*tanh(k[2]*xi-k[3]*t);
yeqn := y = eta+l[3]*tanh(l[2]*eta);

So, p and q are functions of eta and xi, and eta and xi are defined implicitely in terms of x and y.  I have the differentiation correct, but I cannot plot this in terms of x and y.  I want to be able to say something like:

implicitplot({U,xeqn,yeqn},x=-5..5,y=-5..5);

where t, and all the constants have been subbed in. 

How can I do this?

Thanks,

Ryan