Sorry for delay. There is no Open API for custom builds at this time. If you really need it, you can try to do the following:

// obtain BuildPromotion object from the build configuration

BuildPromotionEx promo = ((BuildTypeEx)buildType).createBuildPromotion();

// use setUserBuildParameters() to set custom parameters to build promotion (note you cannot remove parameters, just override them and add new)

promo.setUserBuildParameters(customParams);

// use setCustomArtifactDependencies() if you need to override artifact deps

promo.setBuildComment(user, comment); // this sets comment to the build

// after that you can add promotion to the queue

promo.addToQueue("my custom build");

If you want to build on a specific change, this may not be easy because revisions need to be calculated properly.