The difficulty in providing supplemental medications to cycles stems from the relatively small amount of data there is on anabolic steroids, especially those that are not used medically. Most of the information you read here is scrapped together by body builders using first hand experience, backed up by very basic and sometimes dated clinical studies. Trenbolone is not indicated for human use, so there isn't much information on its effects in the human body. We can speculate as to why it causes "tren cough" and why it ruins your cardiovascular endurance, but there isn't much science backing it up.
That being said, if trenbolone is a leukotriene agonist, then yes singulair would definitely help by countering the activation of these receptors in the lungs. However we have no way of proving this without clinical data, and anything you might read on forums about it is from firsthand experience combined with guesswork.