multinomial logistic regression with equality constraints?

I'm interested in doing multinomial logistic regression with equality 
constraints on some of the parameter values.  For example, with 
categorical outcomes Y_1 (baseline), Y_2, and Y_3, and covariates X_1 
and X_2, I might want to impose the equality constraint that

   \beta_{2,1} = \beta_{3,2}

that is, that the effect of X_1 on the logit of Y_2 is the same as the 
effect of X_2 on the logit of Y_3.

Is there an existing facility or package in R for doing this?  Would 
multinomRob fit the bill?

Many thanks,

Roger

Roger Levy                      Email: rlevy at ucsd.edu
Assistant Professor             Phone: 858-534-7219
Department of Linguistics       Fax:   858-534-4789
UC San Diego                    Web:   http://ling.ucsd.edu/~rlevy