You should absolutely be able to do it... I have done it myself before when testing PSUs. Not with a riser but by plugging the video card into a PSU I had on a work bench. It is how I figured out, without a voltmeter, that one of my PCIe plugins wasn't putting out power.
The slot is going to give the card its first 75w of power and the video card would simply draw the remainder of what it needed from the PSU.
My bigger concern is that you will wind up eating up a lot of extra desk space in order to have a place to mount the PSU and graphics card like an ITX bench or something of that nature.
Not recommending this one... just an example:
https://www.amazon.com/Overclock-Ver...7-93deae8f9840
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To be real... you would probably be better served by buying a motherboard and case then transferring the internals (CPU, Memory/RAM, Storage, other drives) of the Optiplex into a standard form factor. From there just install your new video card and off you go.
If you have the room to spare... then by all means mount the PSU and the GPU onto a test bench, connect the ribbon cable, and call it a day.