How to remove Adafruit Pro Trinket bootloader flashing startup sequence
You could take the Pro Trinket bootloader source code and tweak it to your needs, burn it, and make sure you have patience and debugging skills.

From the author's (Frank Zhao) source code, it seems that this is the 10 seconds loop you are referring to, located in main.c, line #456:

if ( ((usbHasRxed != 0) &amp;&amp; (timeout &gt; USBBOOTLOADER_TIMEOUT)


The USBBOOTLOADER_TIMEOUT variable which is defined to 10 at the top of that file, line #70:

#define USBBOOTLOADER_TIMEOUT 10


and is progressed after 'roughly' 1 second, at line #496:

// roughly 1 second
#if (F_CPU == 12000000)
    if (t1ovf &gt; 183) {
    #elif (F_CPU == 16000000)
    if (t1ovf &gt; 244) {
    #endif
        t1ovf = 0;
        timeout++;
    }