If the main reason you want access to the object is to somehow customize it before it is plotted, instead consider that it is possible to write so called hooks:
def hook(plot, element):
# The bokeh/matplotlib figure
plot.state
# A dictionary of handles on plot subobjects, e.g. in matplotlib
# artist, axis, legend and in bokeh x_range, y_range, glyph, cds etc.
plot.handles
hv.Curve(df, 'x_col', 'y_col').opts(hooks=[hook])