How would one send radio signals from Arduino at a non-standard frequency
The standard 433MHz senders obviously don't work, since they have a totally different frequency.

I think &quot;433 MHz&quot; is used as a band name, with 433 MHz actually being at the lower limit of a band that covers approximately 40 MHz. Your garage door opener will likely be referred to as a &quot;433 MHz garage door opener&quot;, even though it operates at 447.54 MHz.
For your experiments, you might try the HC-12 module. It's billed as a 433 MHz radio, but in fact it covers 433 MHz to 473 MHz in 100 bands with 400 kHz spacing, so it will cover your desired 447.54 MHz. The link above has some Arduino code examples that might help get you started. And of course there is much more information on this topic if you're interested.