I would not use a BP substitute pellet, but that's me. You are in uncharted territory, and charging it with approximately 3x the recommended weight of a very different powder - even without a bullet - seems unwise, especially just for amusement value.
Why not shoot it with an actual bullet? I built mine as a teenager and have shot it numerous times over the decades, using real BP and a round ball (sometimes patched, IIRC).
If the cost of a pound of FFFG is the issue, the purchase of a Pietta cap & ball revolver more than justifies the Hazmat fee for the powder. You have to buy #10 caps anyway...
Aside: would you be willing to scan and post the instruction sheet? Mine went into the trash decades ago; having a copy for the Big Folder of Owner's Manuals would be nice.