you can add another scatter_ternary and fill the space in between the lines with fill="toself"to achieve what you want. You can then use hoverinfo or text on that “shape” as you normally would. For example:
import plotly.express as px
import plotly.graph_objects as go
df = px.data.election()
t = [0.0, 0.0, 0.1, 0.2, 0.2]
l = [0.0, 0.3, 0.3, 0.2, 0.0]
r = [1.0, 0.7, 0.6, 0.6, 0.8]
fig = px.scatter_ternary(df, a="Joly", b="Coderre", c="Bergeron")
fig.add_scatterternary(a=t, b=l, c=r, name="shaded area",
mode='lines', fill="toself", text="throwing some shade", showlegend=False)
fig.show()