Sometimes objects not crafted to be weapons nonetheless see use in combat. Because such objects are not designed for this use, any creature that uses one in combat is considered to be nonproficient with it and takes a –4 penalty on attack rolls made with that object. To determine the size category and appropriate damage for an improvised weapon, compare its relative size and damage potential to the weapon list to find a reasonable match. An improvised weapon scores a threat on a natural roll of 20 and deals double damage on a critical hit. An improvised thrown weapon has a range increment of 10 feet.
This is close to what you want, but does not quite work, RAW, because longswords (and other weapons) are not "not crafted to be weapons". You could, however, do this by the RAW with an actual improvised weapon, like a Giant's letter opener, since that meets the requirements needed to be used as an improvised weapon and thus can be compared to different actual weapons when employed differently. Allowing actual weapons to be used as improvised ones seems extremely reasonable and a good way of dealing with the situation most of the time. Anyone using this method would (and should!) indeed acquire a -4 non-proficiency penalty. I would recommend the following feats if you want to avoid such penalties or make regular use of such tactics (none of these except possibly the last one are appropriate for high-op play):
[Improvised Weapons](Net Book of Feats feat)1
Lower the non-proficiency penalty of weapons.
Prerequisite: Base attack bonus +2
Benefit: When you use a weapon that you are not proficient with, or an improvised weapon for which no proficiency is possible, you suffer only a -2 penalty to hit.
Normal: The normal penalty in this situation is -4.
Combat Options (homebrew feat):
Gain additional options in combat when wielding a weapon you are skilled in.
Prerequisite: Weapon Focus
Benefit: Choose two of the following. When you use a weapon you have Weapon Focus with, you may gain these benefits at your option. Invoking or ending a benefit is a free action that can only be done during your turn and only once per turn per benefit.
- the weapon deals bludgeoning damage instead of the weapon's normal damage type(s)
- the weapon deals piercing damage instead of the weapon's normal damage type(s)
- the weapon deals slashing damage instead of the weapon's normal damage type(s)
- you gain a +1 bonus to hit but take a -1 penalty to AC
- you gain a +1 bonus to AC but take a -1 penalty to hit
- you gain a +1 bonus to damage but take a -2 penalty to AC
- you gain a +1 bonus on Reflex saves but take a -5 ft. penalty to speed
- (etc as appropriate in your campaign. We basically run this like Equipment Trick, below, but with different scaling)
Special: You can take this feat multiple times (but see Improved Combat Options). Its effects stack.
Equipment Trick (Pathfinder feat)
You understand how to use equipment in combat.
Prerequisite: Base attack bonus +1.
Choose: a piece of equipment, such as an anvil, boots, a cloak, a heavy blade scabbard, a rope, a shield, or a sunrod.
Benefit(s): You understand how to use the chosen item in combat. You may use any equipment tricks relating to the item if you meet the appropriate trick requirements. If the item would normally be considered an improvised weapon, you may treat it as a normal weapon or an improvised weapon, whichever is more beneficial for you.
Special: You can gain Equipment Trick multiple times. Each time you take the feat, it applies to a new type of equipment.
Of these Equipment Trick is the best by far (both in terms of optimization and in terms of fun) but requires the most work on the part of the GM as there needs to exist a body of tricks with suitable prerequisites for each chosen equipment item.