Here's the BPM code to initialise Danubia in the event "peoples_springtime.15":

option = {
name = peoples_springtime.15.b
ai_chance = {
base = 50
}

change_tag = KUK
activate_law = law_type:law_confederated
activate_law = law_type:law_cultural_exclusion

custom_tooltip = {
text = bpm_danubia
}

every_country = {
limit = { bpm_country_is_in_europe = yes }
post_notification = bpm_1848_danubia
}

This code does not work for some reason. Your country stays as Austria and Danubia is not formed, even you're a republic, leading to the uncomfortable situation of having to establish a "dual-monarchy" (Since you're not a monarchy Hungary is automatically released from the personal union, effectively giving them independence) or crushing the revolt.

I am not a particularly great modder or debugger, but I'll do some research and add a comment if I find something that works.