Recently, as you no doubt know, I've been beating up on the use of the term "capital" in phrases like "cultural capital" and to a lesser extent "social capital". One desperate last move that their defenders try to deploy is the old, "its only a metaphor" ploy, as though that was some kind of answer. Rather than say, "well that's all right then" I want to take this manoeuver seriously and ask: "so what do you expect your metaphor to accomplish?".
It turns out that there is an old discussion in the philosophy of science that is relevant. I can do little better than quote from Stephen Toulmin's The Philosophy of Science pp35-36:
"When for instance, we say that someone's eyes swept the horizon, the ancient model of vision as the action of antennae from the eye is preserved in our speech as a metaphor; but when we talk of light travelling our figure of speech is more than a metaphor. Consequently, when people say that to talk of light travelling in some sense reflects the nature of the world in a way in which to talk of eyes sweeping the horizon does not, they have some justification. For to say the 'Light travels' reflects the nature of reality, in a way in which 'His eyes swept the horizon' does not, is to point to the fact that the latter remains at best a metaphor. The optical theory from which it came is dead. Questions like 'What sort of broom do eyes sweep with?' and 'What are the antennae made of?' can be asked only frivolously. The former does more: it can both take its place at the heart of a fruitful theory and suggest to us further questions, many of which can be given a sense in a way in which the question suggested by 'His eyes swept the horizon' never could."
"How did he use his cultural capital as collateral for a bank loan?" has roughly the same status as: "What sort of broom do eyes sweep with?" It's obvious that you can't use intangible cultural resources as security against a loan because they aren't capital in a sense that any financial institution would understand. Valuable as they are to you, your cultural resources (unless they are physical objects like paintings, sculptures, opera houses) are not in themselves transferable to others. "Cultural capital" is a metaphor with nowhere to go because it doesn't, as Toulmin puts it, "take its place at the heart of a fruitful theory".