In talking with my GP a few years ago, she explained that drugs like Viagra can help with the physical part of getting an erection, but a hefty part of arousal is mental and the drugs will not change that aspect. Viagra and Cialis are vasodilators rather than hormonal in nature, so they should not conflict with HRT but may not defend against physical atrophy resulting from HRT; as long as you still physically *can* get an erection, the drugs may help it happen.
Note that erectile dysfunction drugs can be dangerous if you have problems with your heart, bood pressure issues, or a host of other cardiopulmonary-related conditions. If your doctor advises against them, don't use them!