Babaco is believed to be a hybrid between the mountain papaya, Vasconcellea pubescens crossed with the culinary papaya, Carica pubescens.
Most of North America and most of EU are too cold to grow Babaco tree outdoors. That is the likely reason for the near total silence on this hybrid mountain papaya. Because one parent is the mountain papaya, the babaco tolerates cool weather in winter down to near a frost.
The other reason you are not getting answers is that papaya (genus Carica) ans mountain papaya (genus Vasconcellea) have a pachycaul growth habit that is not conducive to bonsai. Like plumeria, desert trees, some palm trees and other species that do not have many branches, Babaco is a poor choice for traditional bonsai. The coarse branch structure, with thick trunks with very few branches, and large leaves arranged more like palm fronds, only at the ends of branches, the fine twiggy growth that is typical of a traditional deciduous bonsai simply is not possible. Pachycaul means there is a layer of spongey tissue under the bark layer that is for storing water. This means no fine twigs, all twigs will be thick, like thumbs, rather than fine twigs. Very drought tolerant, so easier horticulture but not good for creating a tree image.
The name bonsai is Japanese and derives from the character for a tray and the character for a pot. Written Classical Japanese is symbolic, rather than phonetic using about 90% of the same characters (symbols) as Classical Chinese. Hence the term "characters" instead of "syllables" or "letters". To be considered "bonsai" a tree needs to be grown in a pot or tray. The tree in the pot should evoke the emotional response one would feel from a scene in nature. It is abstract, and like art versus craft, the goal is the artistic statement. A tree merely pruned to keep it short is NOT bonsai. Bonsai has a design component that is not present in gardening for dwarf fruit tree production.
If you want to apply pruning techniques to a tree that is growing in the ground, there is a parallel Japanese art call Niwaki in English speaking literature. Here pruning and training techniques are applied to trees growing in the ground. Techniques are largely similar to bonsai, except most branch training is with guide wires rather than wrapping branches with wire as in bonsai. Many gardening forums have sub-forums devoted to Niwaki and Japanese gardening.
I do not think papaya will respond very well to container growing. Your best bet with babaco is to grow it in the ground with pruning techniques lifted from Niwaki to keep it less than 4 meters tall.