We have printed on 50/50 garments. Different inks do not perform the same.
The R&H inks we used to use did a decent job on most 50/50 garments- though depending on the shirt some looked more faded or muted than on 100% cotton. Especially touchy for some reason were the gray shirts. They washed pretty well but not as well as 100% cotton. We did lots of hoodies that were 80/20- they almost as good as 100% cotton prints. Higher poly content meant additional cure time- we usually cured them twice.
I have not used DuPont ink myself but I have wash tested a bunch of shirts from other people. The 50/50s I have seen in DuPont ink looked considerably worse than 100% cotton- and DuPont in general does not wash as good as the R & H did by a long way. However not having it in my own machine I have not experimented with it.
The DTGInks that I am using now do really well on shirts with poly content. So far I have not seen much loss in vibrancy of the intial print or loss in wash testing. If it is more than 80/20 we cure it twice (350 for 60 seconds lightest pressure, then repeat). For 100% poly, I have seen it done but have not yet tried it. It requires slightly different technique, but no pretreatment.
http://www.t-shirtforums.com/direct-garment-dtg-inkjet-printing/t51115.html
Another alternative is to use one of the pretreatments available before printing on poly content shirts. I would suggest to everyone on any substrate- do your own wash tests so that you will be comfortable with the quality you are delivering to your customers.