Greetings SirLancelotX.
To minimize loss of muscle mass, ensure your protein intake is at 1gm per pound of bodyweight. From there, manipulate your daily calorie intake via adjustment of carbs and fat.
Your strategy of eating more on lifting days and less on non-lifting days sounds very good. Might even be termed a Targeted Ketogenic Diet (TKD), depending on how much limitation you place on carbs. At the end of the day, calorie balance is what determines weight-gain/loss, but there are tricks you can employ to make this work in your favour.
Should you wish to take the TKD approach, stick to the leanest of meats (there will still be some fat in these) with salads for all meals except those immediately preceding and succeeding your workouts. For these pre/post-workout meals, take 20-30gm protein, preferably from whey, with 20-30gm glucose 15 minutes before and immediately after working out. If you have a solid meal available after working out (grilled meat plus white bread, baked potato, rice, pasta, a high GI carbohydrate etc) then you might wish to omit the post-workout shake.
In essence, keep your meals carb and fat controlled, low in GI if possible, and indulge in high GI carbs just prior to and after training to optimize your workout peformance. Keep a food log, and shoot for a weight gain of no more than 2lbs per week. Any more than that and you'd be gaining fat. It would also be good to measure your bodyfat every so often.
Should you wish to just cut calories, then every meal can have carbs. Some experience better appetite control and satiety on a ketogenic-type diet. This is something you have to find out.
Another alternative is to NOT cut calories, and instead, consume slightly (100-200 calories) above maintenance. In addition to this, engage in some form of endurance-type work on non-lifting days. It is suggested that up to 40 minutes of walking on an incline (treadmill or uphill) be performed 2-3 times per week. This increased food intake and increased activity can result in an increase in metabolic rate resulting in greater body composition modification without the associated evils of being direct calorie deficit without extra activity. In short, [dietary calorie deficit & no increase in activity = calorie deficit] isn't as good as [slight dietary calorie surplus & endurance exercise = calorie deficit].
In hindsight, why worry? You already are getting the modelling contracts, and perhaps, too much muscle might be detrimental to the pretty boy look?
Still, keep us updated on your progress and how you achieved it.
Godspeed, happy HSTing and happy modelling
