Hi.
The supplies the British and French could send through the Black Sea ports would be too little too late to save Poland by themselves.
That said, Romania entering the war about the same time as Britain and France (this implies no refusal of Romanian intervention by the Poles themselves) could, IMHO, lead to one of these scenarios:
1. Germany quickly smashes Poland and goes on to defeat Romania in one or two quick and decisive border engagements, leading to a change in the Romanian leadership and a cease-fire. Could possibly be coupled with attacks on Romania by Hungary, Bulgaria and/or the SU. War goes almost as OTL from then on. (unless the oilfields & refineries sustain considerable damage, in which case Barbarossa is far less successful)
2. Germany quickly smashes Poland and goes on to invade Romania. However, the large distances, rugged and even mountainous terrain, horrible roads and the sizeable Romanian army lead to Germany suffering ireplaceable losses due to combat and/or attrition. This enables to Anglo-French to halt to German invasion in May 1940 by the skin of their teeth. Entente victory in late 1941 at the latest.
3. Reasoning that it's not too late to launch an offensive, French forces attack the Siegfried line in force before the Germans have overrun all of Poland. This either leads to:
a. they are reasonably successful, forcing the Germans to halt their offensive in the east. Entente victory within a few months as the German war-effort collapses
b. they fail miserably, leading to either 1) or 2)
So, what do you think ? Plausible ?