The boot loader doesn't determine that. Assume your actual adversaries understand the platform/technologies better than you, they have more time and more resources. Figure you are matching wits with Einstein or Tesla, or both at once.
If the micro-controller can figure it out how hard can it be for anyone else?
Are you talking about your own loader, or the system loader (BOOT0=High)?
The memory at 0x1FFxxxxx other than the options and OTP is generally not writable.
On the H7 I think you can specify a start block for the vector table, but not sure how useful that is.
