How do I apply boundary conditions to coefficients estimated in VAR model using VGXVARX / VGXSET / MVREGRESS?

For specific reasons I want to limit the estimated coefficients in a 1st order VAR to a specific value, lets say 0.9. I assume that it must be possible, does anyone know how?

To be more specific, I know that it is possible to set various values to zero by using boolean matrices, but I want to limit the estimated coefficients in the AR matrix to a specific value.

Spec                            = vgxset('n', size(Z,2), 'ARsolve', mat);
[EstSpec, EstStdErrors, LLF, W] = vgxvarx(Spec,Z);

Hope anyone can help me, thanks in advance!