Online 'election' would send same message without the risk
If the pan-democrats want to hold a de facto referendum, why don't they organise their own de facto election online and/or on paper, rather than resigning?
This way, they could still mount a campaign, target the issues, mobilise voters and claim a de facto mandate from the results, without risking their veto power. They could all act in a united way; no one would need to resign.
Other advantages include that the pan-democrats would control the parameters for the de facto election. They could run it to UN standards and bring in United Nations monitors and control electoral fraud. They can also involve people other than registered voters, if they wish, and obtain an even broader mandate than would be possible under the formal voting system. The exercise could serve as a possible model election.
The League of Social Democrats is very good at dramatic gestures and political theatre, capturing the attention of the masses, fighting for the grass roots and effectively highlighting the government's flaws, but the resignation plan, although a clear and strong protest at the government's duplicitous reform proposals, is also a dangerous path, a potential disaster and a great Christmas present for the government, which probably can't believe its luck and is exploiting the situation and goading the pan-democrats on.
The pan-democrats don't have to prove their courage by resigning - they've already demonstrated it in many other ways over many years.
They are the voices of democracy and everyone knows it.