Decided i may as well give my suggestion on how to get this without any modifications to raws.
Firstly, gen a world with lots of megabeasts. If you want very few rejections, try using this one, should get you a exactly 5 dragons.
[WORLD_GEN]
[TITLE:Dragons]
[DIM:33:33]
[EMBARK_POINTS:512]
[END_YEAR:20]
[BEAST_END_YEAR:50:80]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:1:310:102:102]
[RAINFALL:10:100:80:80]
[TEMPERATURE:30:75:51:51]
[DRAINAGE:0:100:51:51]
[VOLCANISM:0:100:51:51]
[SAVAGERY:0:100:100:100]
[ELEVATION_FREQUENCY:1:1:1:1:1:1]
[RAIN_FREQUENCY:1:1:1:1:1:1]
[DRAINAGE_FREQUENCY:1:1:1:1:1:1]
[TEMPERATURE_FREQUENCY:1:1:1:1:1:1]
[SAVAGERY_FREQUENCY:1:1:1:1:1:2]
[VOLCANISM_FREQUENCY:1:1:1:1:1:2]
[POLE:NORTH_OR_SOUTH]
[MINERAL_SCARCITY:250]
[MEGABEAST_CAP:20]
[SEMIMEGABEAST_CAP:2]
[TITAN_NUMBER:1]
[TITAN_ATTACK_TRIGGER:80:0:100000]
[DEMON_NUMBER:20]
[NIGHT_TROLL_NUMBER:10]
[BOGEYMAN_NUMBER:10]
[VAMPIRE_NUMBER:10]
[WEREBEAST_NUMBER:10]
[SECRET_NUMBER:20]
[REGIONAL_INTERACTION_NUMBER:20]
[DISTURBANCE_INTERACTION_NUMBER:20]
[EVIL_CLOUD_NUMBER:10]
[EVIL_RAIN_NUMBER:10]
[GENERATE_DIVINE_MATERIALS:1]
[GOOD_SQ_COUNTS:0:2:0]
[EVIL_SQ_COUNTS:2:32:0]
[PEAK_NUMBER_MIN:0]
[PARTIAL_OCEAN_EDGE_MIN:1]
[COMPLETE_OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:2]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:6:6]
[PERIODICALLY_ERODE_EXTREMES:1]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVERN_LAYER_COUNT:3]
[CAVERN_LAYER_OPENNESS_MIN:70]
[CAVERN_LAYER_OPENNESS_MAX:100]
[CAVERN_LAYER_PASSAGE_DENSITY_MIN:0]
[CAVERN_LAYER_PASSAGE_DENSITY_MAX:25]
[CAVERN_LAYER_WATER_MIN:5]
[CAVERN_LAYER_WATER_MAX:15]
[HAVE_BOTTOM_LAYER_1:1]
[HAVE_BOTTOM_LAYER_2:1]
[LEVELS_ABOVE_GROUND:5]
[LEVELS_ABOVE_LAYER_1:5]
[LEVELS_ABOVE_LAYER_2:1]
[LEVELS_ABOVE_LAYER_3:1]
[LEVELS_ABOVE_LAYER_4:1]
[LEVELS_ABOVE_LAYER_5:2]
[LEVELS_AT_BOTTOM:1]
[CAVE_MIN_SIZE:5]
[CAVE_MAX_SIZE:25]
[MOUNTAIN_CAVE_MIN:1]
[NON_MOUNTAIN_CAVE_MIN:3]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:10]
[TOTAL_CIV_POPULATION:15000]
[SITE_CAP:68]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:60:100:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
Also produces a decent amount of goblins and 2 volcanoes
To use this, the easiest way is to copy it and to go into dwarf fortress folder -> data -> init -> worldgen.txt open that and paste it at the bottom. When you go into "design new world with advanced parameters" it should be there, called "dragons".
firstly ensure your world has dragons by going into play -> legends -> Historical figures
You should get something like this:
Now we've confirmed we have several dragons which arn't dead we need to locate one.
Personally i suggest using "legends Viewer".
If you use the starter pack (which i highly recommend):
http://www.bay12forums.com/smf/index.php?topic=126076.0 then you already have it.
If not:
http://www.bay12forums.com/smf/index.php?topic=72702.0Now we follow the instructions it gives in its readme which boils down to:
1. in legends mode in DF press x (for XML dump) nothing "appears" to happen. Thats fine.
2. Press p for map/gen info
3. press d then select standard biome + site map.
4. Open legends viewer and click the ... next to Legends XML and select the xml which can be found in the DF folder (should be named something like: region1-00020-01-01-legends.xml)
This should load up everything. Next we need to locate the dragons!
Select historical figures and filter by dragons. select one and see where he lives!
In my case:
Related Sites
Wealthstax, Lair (0)
Click on the Lair name (in this case Wealthstax), and you should get an image like this:
Now we have the location we can find it!
Bare in mind this does not give the precise tile the dragons is located. So i suggest making a copy of your save file on your desktop, making an 8x8 embark in a corner (my computer which is pretty decent failed to load a 16x16 embark in 5 mins) and see if it's there. It will say you've discovered a lair should you have chosen the correct location. If you have not, embark again (as shown):
Once you have found the lair, you can move your old save back and embark properly with a normal 3x3 or whatever you want embark. Getting it right may be a little iffy, but you should have a decent idea of where it is in the 8x8.
Well, that's how i'd do it anyway.
Zuglars suggestion would probably be the easiest way of doing it.