Sorry, that's ridiculous in every possible way. The inside of a capsule cannot be pressed into service as a hypergolic tank. You just turned Dragon inside out.
Exactly, you turn the Dragon inside out. It's a specialty variant for Mars transport missions
And why not? It is airtight, and can be pressurized -- exactly what the Hypergolic tanks needs to be. Sure bare aluminum is not compatible with a corrosive oxidizer, but neither is the aluminum in a Titan rocket's body. That's why you coat it on the inside with an appropriate barrier material.
In fact, not doing so is ridiculous. Let's look at the Dragon Spacecraft as is...
(1) You have a spacecraft whose dominant volume is a pressure vessel which is useless in a Mars mission because there is no reason anything going to mars is going to need to be pressurized, much less enough space for 7 humans plus life support equipment. So you basically have a spacecraft which is 75% useless volume and mostly dead weight.
(2) There isn't enough space for adequate propellant in the Dragon for Mars Capture and/or orbit insertion if you do not use the pressure vessel as a fuel tank.
(3) If you do not use on-board Hypergolics and the Dragon's own thrusters for Mars Capture and Orbit insertion. You are going to need to use either the upper stage or a new trans Mars propulsion stage for it. The former is not going to work because the Falcon upper stage uses LOX as the oxidizer and LOX is a cryogenic liquid. You can't keep that from boiling off during the 6~9 month trip. That is why all the interplanetary missions do not use LH2/LOX or RP-1/LOX propulsion for destination burns. They use storable liquids or solids. If you don't use the upper stage then you need a new propulsion stage for Mars Capture. That's a heck of a lot more work than improvising with the Dragon itself, and if you are going to essentially build something from the ground up why even use the Dragon with its big fat pressure vessel? Why not just build a spacecraft like a typical Mars probe?
This is the only way a Dragon based Mars transit makes sense.NOTE: ALL THE DELTA Vs add up. You eliminate the need for worry about a Mars Capture stage in the mission. You have plenty of propellants to insert youself into whatever desired orbit right down to LMO. You have enough to land with after an aerobraking re-entry.