Altough I didn`t try it, IMO - pentode can be implemented (little problem with g2 voltage, but some people use pentode for "upper" tube in mu-follower, or like active load). But, IME - gain of pentode depends very little of "upper" tube. A~gm1*Rl, where gm1 is gm of the "lower" tube. Composite model for "penscode" - I have no idea, maybe SPICE it first and see what PC says...