I figured it out; it turns out that I can’t create a new visual.Slider manually in JS (I can do so in PsychoPy), but I can manually set the existing slider properties depending on my conditions table:
code in PsychoPy:
qn_slider = visual.Slider(win=win, name='qn_slider',
size=(0.025, slider_height), pos=(0.1, 0), units='height',
labels=demolabels, ticks=demoticks,
granularity=1, style=(respStyle),
color='LightGray', font='HelveticaBold',
flip=False, labelHeight = .04)
code in JS (with a slider component also added in the builder):
qn_slider.size = [0.025, slider_height];
qn_slider.labels = demolabels;
qn_slider.ticks = demoticks;
qn_slider._needVertexUpdate = true;
qn_slider._buildSlider();