Can "when verb+ing" refer to an activity in the past?

Subjectless preposition phrases using gerund-participles have no tense and can be interpreted as referring to present, past or future time. The time being referenced is understood by reference to the time referred to in the main clause, sometimes in combination with a perfect construction in the preposition phrase.

• Before taking my aspirin, I shall drink a glass of water.
• Before taking my aspirin, I drink a glass of water.
• Before taking my aspiring, I drank a glass of water.

You have a dangling modifier there. You could fix it by saying

When designing the product, they paid attention to . . .

Just follow the template of your first sentence.