Dominica is a small country but has surprisingly lot of highways too. And shares something with Germany - no speed limits. Hitching works and people are friendly. Watch out for the cars that have TX-plates. They´re taxis and might want money.
Hitching in St Lucia can´t really be recommended. Such culture doesn´t really exist there. Local minibuses run frequently and people just don´t understand why someone wants to hitch. This can be a problem on Sundays when local minibuses don´t run on every route.
Border crossing: This truly is a country where you can´t get in without an onward ticket. However, if you´re coming by a ferry from Martinique, they´ll force you to buy an open return ticket. But if you then will fly somewhere from St Lucia, you can cash the ticket back when you show the air ticket. And as a matter of fact, you can even sell your open ferry ticket. Good to know is also that the catamaran from Martinique is not the only alternative. From the town of Marin in Martinique you´ll find a little water bus coming to Castries. Not remarkable difference in the price. No ship connection to St Vincent though the country can be seen from the southern tip of St Lucia.