Substitute for miso paste

There really is no substitute. I'd look for a substitute recipe instead.

Search "miso substitute" in the box above right, putting "Hotline" in where you see recipes. There have been a number of good suggestions in the past.

You might try hoisin sauce if you have it -- very different, sweet rather than salty, but it would have color and body.

There really is no substitute for miso--it has a flavor all its own. But you can replace the umami notes that miso provides. I make Asian-style dressings a lot (for salads, soba noodle dishes, stir fries, tofu dishes, etc.), and I almost never use a recipe.
It's fun to play around with flavors. I try to keep a good brand of shoyu on hand, ponzu sauce, fermented black bean sauce, fish sauce (Red Boat is amazing), rice vinegar, sesame oil, chili oil, etc. At that point, the key is to balance the flavors (I don't use all those condiments in the same sauce). For salty and umami, use the shoyu and/or fish sauce. For sweet, use ponzu. For hot, use chili oil (or even Tabasco in a pinch). For sour, use lime or lemon juice. The sesame oil provides a nice, rich flavor. Adjust according to your tastes and preferences.
For recipes, there's a nice Thai vinaigrette in the Joy of Cooking: 1/2 c lime juice, 3 Tbsp fish sauce, 1 Tbsp soy sauce, 1 1/2 tsp sugar, 1/4 tsp red pepper flake or 1 tsp chili oil, salt and pepper to taste, 1/2 c veg oil.
Also see 101cookbooks.com. Heidi Swanson has a lot of dishes that involve Asian dressings, and her recipes are usually reliably good.