Ok, I've partly answered my own question. Scribble can be invoked by bringing up watchOS's built-in text input controller, provided that you invoke it using an array of suggestions.
This brings up the provided list of suggestions, as well as a dication input option and a Scribble input option:
[self presentTextInputControllerWithSuggestions:@[@"Item 1", @"Item 2"] allowedInputMode:WKTextInputModePlain completion:^(NSArray * _Nullable results) {
/
}];
This brings up only the dictation input option.
[self presentTextInputControllerWithSuggestions:nil allowedInputMode:WKTextInputModePlain completion:^(NSArray * _Nullable results) {
/
}];
Long story short, Apple's built in text input controller isn't robust/customizable enough for my app so I ended up having to build my own. At the very bottom of my custom controller are two buttons; one to activate dictation input and one to activate Scribble input. The dictation input option works great using second code snippet. However I need to find a way to link up the other button to Scribble, without confusing my users by presenting them with Apple's text selector control on top of my text selector control.