If you only want a little magma, impulse ramps work fine. They have the reliability issues you observed - practically impossible to get a moving cart to pick up fluid in 6/7 magma, no escape if all tiles are 7/7. When all you want is enough magma for some workshops, it's still an adequate method, the dodgy reliability starts to hurt when you want a steadier supply.
For long-term sustained reloading with magma at zero power, you need the single-tile trench: a ramp in a one-wide pit, with track connection only to walls. The cart will treat the tile as flat floor and thus doesn't need to "climb" outside. Magma friction is still so high that ordinary cart movement through the tile is not feasible. It works best with a collision system - impulse ramp on the (non-magma) tile before/above the magma trench, pointing into the magma.
You have to start with a cart sitting in the magma already. An arriving cart will push the cart in the magma out of the trench (must be at moderate speed, if the trenched cart gets accelerated too much it'll drop its load). Then the cart on the ramp will roll off, but gains only a single step worth of ramp acceleration and gets stopped inside the trench by the checkpoint effect. Since it stands still, it will take on magma when the tile is filled to 7/7
or 6/7.
This system is very reliable and can be automated:
http://mkv25.net/dfma/movie-2733-multi-cartmagmadeliveryBTWPS: You can keep pets and guests off minecart tracks by blocking them off with statues. Carts travel through statues just fine, creatures don't (apart from creatures in minecarts, of course). Unwalkable workshop tiles fulfill the same purpose, but i find statues more economical - i have more than enough crummy statues in every fort.