There may be a way to send CAT commands for that using a Macro that will need to be programmed.
First though, some questions for you.
How do you have the TX and RX audio connected from the RigPi to the FT-450D?
Are you using the DATA mini-DIN6 connector or something different?
Are you wanting to have the same function as pressing the DSP button on the front of the radio?
Does the audio already have DSP enabled and you want to be able to turn that off and then on, or is DSP missing and you wish to have DSP?
Reading the Yasesu CAT guide for the FT-450D, I am left with the idea that a Mic UP or DN button could be assigned the role of the DSP button.
Then the Macro could be to press the UP or DN button remotely.
73 - Mark - K8MHW
'A RigPi Enthusiast!' - I am not otherwise affiliated with MFJ or RigPi