I had some problems with this as well, I managed to fine tune the settings and got it working for both the cargo and fuel. It's pretty accurate (till 0.1% MAC) but as soon as I start adding passengers things go wrong.
I use the PMDG 737-800NGX WL with mixed class (first and coach) but I'm not yet sure how PMDG is distributing the pax since their are three holds.
Apparently the distribution is 28,17% + 43,66% + 28,17% but I'm waiting on both PMDG and other forums to confirm this distribution is applicable.
It appears to me that this distribution above does not look to your pax configuration of PMDG in the FMC since it would be regardless of how many pax you put in which class, and just looks to the total pax and divide them.
Anyway: according to me it is definitely possible have correct values between PMDG and TOPCAT, you do have to enter your values manually in the FMC.
Keep in mind, I can share my configuration file for TOPCAT but I configured it for KGS.
Should you have any questions, please contact me.
Sander De Prycker